Move back to Travis's VM infrastructure for efficiency.

Document that a commit is not a descendant of itself

Use SOCK_CLOEXEC when creating sockets

Static linking for bundled deps

The block in the script installs the packages if we're _not_ on Precise. This
was dropped in c17c3f8a ("travis: drop support for Ubuntu Precise") in error.

Our bundled deps are being built as simple static libraries which are
then linked into the libgit2 library via `TARGET_LINK_LIBRARIES`. While
this works for a dynamically built libgit2 library, using this function
to link two static libraries does not have the expected outcome of
merging those static libraries into one big library. This leads to
symbols of our bundled deps being undefined in the resulting libgit2
archive.

As we have bumped our minimum CMake version to 2.8.11, we can now easily
make use of object libraries for our bundled dependencies. So build
instructions are still self-contained inside of the dependency
directories and the resulting object libraries can just be added to the
LIBGIT2_OBJECTS list, which will cause them to be linked into the final
resulting static library. This fixes the issue of undefined symbols.

There are two locations where we check whether CMake supports
`TARGET_INCLUDE_DIRECTORIES`. While the first one uses `VERSION_LESS
2.8.12`, the second one uses `VERSION_GREATER 2.8.11`, which are
obviously equivalent to each other. It'd still be easier to grep for
specific CMake versions being required for some features if both used
the same conditional mentioning the actual target version required. So
this commit refactors these conditions to make them equal.

As we have bumped our minimum CMake version to 2.8.11, we can now
unconditionally make use of object libraries. So remove the version
check for the git2internal object library and always use it.

Our current minimum CMake version is 2.8. This version does not yet
allow us to use object libraries (introduced in 2.8.8) and target
include directories (introduced in 2.8.12), which are both mechanisms we
want to use to fix some specific problems. We previously were not able
to bump our CMake version to a version supporting object libraries
because Ubuntu Precise only had CMake version 2.8.7 in its repositories.
But due to Precise being end of life now, we shouldn't need to honor it
anymore. A current survey of some of the more conservative distributions
brings up the following versions of CMake:

- CentOS 5: 2.6.2
- CentOS 6: 2.8.12.2
- Debian 7: 2.8.11
- Fedora 23: 3.3.2
- OpenSUSE 13.2: 3.0.2
- Ubuntu Precise: 2.8.7
- Ubuntu Trusty: 2.8.12

The only two outliers here are CentOS 5 and Ubuntu Precise. CentOS is
currently unsupported due to our minimum version being 2.8 and Ubuntu
Precise is not maintained anymore. So the next smallest version
supported by all major distributions is 2.8.11. While this does not yet
support target include directories, it at least enables us to use object
libraries. So this becomes our new minimum required version.

Distinguish variables keeping track of our internal libgit2 sources and
the final objects which shall be linked into the library. This will ease
the transition to use object libraries for our bundled dependencies
instead of linking them in.

Ubuntu Precise is end of life since April 2017. At that point in time,
Precise was still the main distro on which Travis CI built upon, with
the Trusty-based images still being in a beta state. But since June
21st, Trusty has officially moved out of beta and is now the default
image for all new builds. Right now, we build on both old and new images
to assure we support both.

Unfortunately, this leaves us with the highest minimum version for CMake
being 2.8.7, as Precise has no greater version in its repositories. And
because of this limitation, we cannot actually use object libraries in
our build instructions. But considering Precise is end of life and
Trusty is now the new default for Travis, we can and should drop support
for this old and unmaintained distribution. And so we do.

Reproducible builds

diff: cleanup hash ctx in `git_diff_patchid`

Fix AppVeyor build failures due to CRTDBG linking issue

In order to cover a wider range of build environments, add two more jobs
which build and test libgit2 on Visual Studio 14 2015.

AppVeyor currently does provide three standard build worker images with
VS2013, VS2015 and VS2017. Right now, we are using the implicitly, which
is the VS2015 one. We want to be more explicit about this, so that we
can easily switch build images based on the job. So starting from this
commit, we explicitly set the `APPVEYOR_BUILD_WORKER_IMAGE` variable per
job, which enables us to choose different images.

To be able to test a wider range of build configurations, this commit
also switches the jobs for VC2010 over to use the older, VS2013 based
images. As the next commit will introduce two new jobs for building with
VS2015, we have then covered both build environments.

Also, let us be a bit more explicit regarding the CMake generator.
Instead of only saying "Visual Studio 10", use the more descriptive
value "Visual Studio 10 2010" to at least avoid some confusion
surrounding the versioning scheme of Visual Studio.

When the MSVC_CRTDBG option is set by the developer, we will link in the
dbghelper library to enable memory lead detection in MSVC projects. We
are doing so by adding it to the variable `CMAKE_C_STANDARD_LIBRARIES`,
so that it is linked for every library and executable built by CMake.
But this causes our builds to fail with a linker error:

```
    LINK: fatal error LNK1104: cannot open file 'advapi32.lib;Dbghelp.lib'
```

The issue here is that we are treating the variable as if it were an
array of libraries by setting it via the following command:

```
    SET(CMAKE_C_STANDARD_LIBRARIES "${CMAKE_C_STANDARD_LIBRARIES}"
        "Dbghelp.lib")
```

The generated build commands will then simply stringify the variable,
concatenating all the contained libraries with a ";". This causes the
observed linking failure.

To fix the issue, we should just treat the variabable as a simple
string. So instead of adding multiple members, we just add the
"Dbghelp.lib" library to the existing string, separated by a space
character.

After initializing the hash context in `git_diff_patchid`, we never
proceed to call `git_hash_ctx_cleanup` on it. While this doesn't really
matter on most hash implementations, this causes a memory leak on Win32
due to CNG system requiring a `malloc` call.

Fix the memory leak by always calling `git_hash_ctx_cleanup` before
exiting.

By default, both ar(1) and ranlib(1) will insert additional information
like timestamps into generated static archives and indices. As a
consequence, generated static archives are not deterministic when
created with default parameters.

Both programs do support a deterministic mode, which will simply zero
out undeterministic information with `ar D` and `ranlib -D`.
Unfortunately, CMake does not provide an easy knob to add these command
line parameters. Instead, we have to redefine the complete command
definitons stored in the variables CMAKE_C_ARCHIVE_CREATE,
CMAKE_C_ARCHIVE_APPEND and CMAKE_C_ARCHIVE_FINISH.

Introduce a new build option `ENABLE_REPRODUCIBLE_BUILDS`. This option
is available on Unix-like systems with the exception of macOS, which
does not have support for the required flags. If the option is being
enabled, we add those flags to the invocation of both `ar` and `ranlib`
to enable deterministically building the static archive.

Clear the remote_ref_name buffer in git_push_update_tips()

features.h: allow building without CMake-generated feature header

In commit a390a846 (cmake: move defines into "features.h" header,
2017-07-01), we have introduced a new "features.h" header. This file is
being generated by the CMake build system based on how the libgit2 build
has been configured, replacing the preexisting method of simply setting
the defines inside of the CMake build system. This was done to help
splitting up the build instructions into multiple separate
subdirectories.

An overlooked shortcoming of this approach is that some projects making
use of libgit2 build the library with custom build systems, without
making use of CMake. For those users, the introduction of the
"features.h" file makes their life harder as they would have to also
generate this file.

Fix this issue by guarding all inclusions of the generated header file
by the `LIBGIT2_NO_FEATURES_H` define. Like this, other build systems
can skip the feature header and simply define all used features by
specifying `-D` flags for the compiler again.

If fetch_spec was a non-pattern, and it is not the first iteration of push_status vector, then git_refspec_transform would result in the new value appended via git_buf_puts to the previous iteration value.

Forcibly clearing the buffer on each iteration to prevent this behavior.

README: Mention Guile-Git bindings.

The script "generate.py" is used to parse all test source files for unit
tests. These are then written into a "clar.suite" file, which can be
included by the main test executable to make available all test suites
and unit tests.

Our current algorithm simply collects all test suites inside of a dict,
iterates through its items and dumps them in a special format into the
file. As the order is not guaranteed to be deterministic for Python
dictionaries, this may result in arbitrarily ordered C structs. This
obviously defeats the purpose of reproducible builds, where the same
input should always result in the exact same output.

Fix this issue by sorting the test suites by name previous to dumping
them as structs. This enables reproducible builds for the libgit2_clar
file.

Fix negative ignore rules with patterns

Submodules with bare repo

-Werror builds for Travis

While it is technically possible to look up submodules inside of a
bare repository by reading the submodule configuration of a specific
commit, we do not offer this functionality right now. As such, calling
both `git_submodule_lookup` and `git_submodule_foreach` should error out
early when these functions encounter a bare repository. While
`git_submodule_lookup` already does return an error due to not being
able to parse the configuration, `git_submodule_foreach` simply returns
success and never invokes the callback function.

Fix the issue by having both functions check whether the repository is
bare and returning an error in that case.

The testcase "submodule::lookup::cached" was declared with a single
underscore separating the test suide and test name, only. As the clar
parser only catches tests with two underscores, it was never executed.
Add in the second underscore to actually have it detected and executed.

When computing negative ignores, we throw away any rule which does not
undo a previous rule to optimize. But on case insensitive file systems,
we need to keep in mind that a negative ignore can also undo a previous
rule with different case, which we did not yet honor while determining
whether a rule undoes a previous one. So in the following example, we
fail to unignore the "/Case" directory:

    /case
    !/Case

Make both paths checking whether a plain- or wildcard-based rule undo a
previous rule aware of case-insensitivity. This fixes the described
issue.

This test is by Carlos Martín Nieto.

Ignore rules allow for reverting a previously ignored rule by prefixing
it with an exclamation mark. As such, a negative rule can only override
previously ignored files. While computing all ignore patterns, we try to
use this fact to optimize away some negative rules which do not override
any previous patterns, as they won't change the outcome anyway.

In some cases, though, this optimization causes us to get the actual
ignores wrong for some files. This may happen whenever the pattern
contains a wildcard, as we are unable to reason about whether a pattern
overrides a previous pattern in a sane way. This happens for example in
the case where a gitignore file contains "*.c" and "!src/*.c", where we
wouldn't un-ignore files inside of the "src/" subdirectory.

In this case, the first solution coming to mind may be to just strip the
"src/" prefix and simply compare the basenames. While that would work
here, it would stop working as soon as the basename pattern itself is
different, like for example with "*x.c" and "!src/*.c. As such, we
settle for the easier fix of just not optimizing away rules that contain
a wildcard.

One of our goals is to have our code free of any warnings. Due to the
recent switch to Ubuntu 14.04 on Travis, the last warning regarding some
preprocessor-magic in the curl-headers has been fixed and as such, the
goal of zero warnings is now reached for Travis CI. In order to avoid
introducing new warnings via pull requests, we can now enable building
with `-Werror` and turn compiler warnings into errors instead, causing
the CI jobs to fail.

This build does so by passing the newly introdcued `-DENABLE_WERROR`
flag to CMake for all Travis jobs.