Refactor citest.sh to enable local testing by developers.

Add citest.ps1 PowerShell script to run the tests.

Simplify the names for the tests, removing the unnecessary
"libgit2-clar" prefix.  Make "all" the new default test run, and include
the online tests by default (since HTTPS should always be enabled).

For the CI tests, create an offline-only test, then the various online
tests.

mbedtls: remove unused variable "cacert"

In commit 382ed1e8 (mbedtls: load default CA certificates, 2018-03-29),
the function `git_mbedtls_stream_global_init` was refactored to call out
to `git_mbedtls__set_cert_location` instead of setting up the
certificates itself. The conversion forgot to remove the now-unused
"cacert" variable, which is now only getting declared to be free'd at
the end of the function. Remove it.

Squash some leaks

Instead of allocating the ciphers_list, make it a static array.  This
prevents us from leaking it or having to manage its memory.

Don't just free the spec vector, also free the specs themselves.

Don't just free the push status structure, actually free the strings that were
strdup'd into the struct as well.

Free the url field when resetting the stream to avoid leaking it.

Add a checkout example

Assorted Coverity fixes

Remove GIT_PKT_PACK entirely

 ignore: improve `git_ignore_path_is_ignored` description Git analogy

alloc: don't overwrite allocator during init if set

If the allocator has been set before we the library is initialised, we would
replace that setting with the standard allocator contrary to the user's wishes.

C90 standard compliance

In attempt to provide adequate Git command analogy in regards to
ignored files handling, `git_ignore_path_is_ignored` description
mentions doing `git add .` on directory containing the file, and
whether the file in question would be added or not - but behavior of
the two matches for untracked files only, making the comparison
misleading in general sense.

For tracked files, Git doesn't subject them to ignore rules, so even
if a rule applies, `git add .` would actually add the tracked file
changes to index, while `git_ignore_path_is_ignored` would still
consider the file being ignored (as it doesn't check the index, as
documented).

Let's provide `git check-ignore --no-index` as analogous Git command
example instead, being more aligned with what `git_ignore_path_is_ignored`
is about, no matter if the file in question is already tracked or not.

See issue #4720 (git_ignore_path_is_ignored documentation
misleading?, 2018-07-10)[1] for additional information.

[1] https://github.com/libgit2/libgit2/issues/4720

While the aim of libgit2 was to conform to C90 code, we never instructed
the compiler to enforce C90 compliance. Thus, quite a few violations
were able to get into our code base, which have been removed with the
previous commits. As we are now able to build libgit2 with C90 enforced,
we can set the C_STANDARD property for our own build targets.

Note that we explicitly avoid setting the C standard for our third-party
dependencies. At least the zlib target does not build with C90 enforced,
and we do not want to fix them by deviating from upstream. Thus we
simply enforce no standard for them.

The mbedtls headers make direct use of the `inline` attribute to
instruct the compiler to inline functions. As this function is not C90
compliant, this can cause the compiler to error as soon as any of these
files is included and the `-std=c90` flag is being added.

The mbedtls headers declaring functions as inline always have a prelude
which define `inline` as a macro in case it is not yet defined. Thus, we
can easily replace their define with our own define, which simply copies
the logic of our own `GIT_INLINE` macro.

While we want to enforce strict C90 mode, this may cause issues with
system provided header files which are themselves not strictly
conforming. E.g. if a system header has C++ style comments, a compiler
in strict C90 mode would produce an error and abort the build. As the
user most likely doesn't want to change the system header, this would
completely break the build on such systems. One example of this is
mbedtls, which provides such header files.

The problem can be worked around by distinguishing between
system-provided and project-provided include directories. When adding
include directories via "-isystem" instead of "-I", the compiler will
skip certain checks and print out less warnings. To use system includes,
we can simply add the "SYSTEM" flag to CMake's `INCLUDE_DIRECTORIES` and
`TARGET_INCLUDE_DIRECTORIES` functions. Note that we have to split the
include directories into two variables because of this, as we definitely
still want to check for all warnings produced by our own header files.

C++ style comment ("//") are not specified by the ISO C90 standard and
thus do not conform to it. While libgit2 aims to conform to C90, we did
not enforce it until now, which is why quite a lot of these
non-conforming comments have snuck into our codebase. Do a tree-wide
conversion of all C++ style comments to the supported C style comments
to allow us enforcing strict C90 compliance in a later commit.

ISO C90 does not specify the `inline` attribute, and as such we cannot
use it in our code. While we already use `__inline` when building in
Microsoft Visual Studio, we should also be using the `__inline__`
attribute from GCC/Clang. Otherwise, if we're using neither MSVC nor
GCC/Clang, we should simply avoid using `inline` at all and just define
functions as static.

This commit adjusts our own `GIT_INLINE` macro as well as the inline
macros specified by khash and xdiff. This allows us to enable strict C90
mode in a later commit.

Delta OOB access

streams: report OpenSSL errors if global init fails

As git_annotated_commit seems to behave like cgit's refish, it's quite
helpful to abstract away "targets" via git_annotated_commit_from_id/from_ref.
As the former is accessible via git_annotated_commit_id, make the latter
also available to users.

The path given to `git_index_add_bypath` is relative to the root of the
repository. That `describe/file` path is relative to the root of the
sandbox directory, hence if I add the missing `cl_git_pass` I rightfully
get an error that `$SANDBOX/describe/describe/file doesn't exist`.

The path is thus changed to be made relative to the repository, which
makes the failure go away and "restore" the test.

Reported by Coverity, CID 1393678-1393697.

By clarifying what detect_caps returns on empty/missing packet, we can
be sure there are actually refs to process. The old code could blindly
dereference `first`, which might have been NULL.

Reported by Coverity, CID 1393614