If we enter the sha1_position() function with "lo == hi",
we have no elements. But the do-while loop means that we'll
enter the loop body once anyway, picking "mi" at that same
value and comparing nonsense to our desired key. This is
unlikely to match in practice, but we still shouldn't be
looking at the memory in the first place.

This bug is inherited from git.git; it was fixed there in
e01580cfe01526ec2c4eb4899f776a82ade7e0e1.

If the signature is invalid but we detect that after allocating the strings, we
free them. We however leave that pointer dangling in the structure the caller
gave us, which can lead to double-free.

Set these pointers to `NULL` after freeing their memory to avoid this.

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.

`sin_port` should be in network byte order.

Fixes #4274

Security fixes for reading index v4

When computing the complete path length from the encoded
prefix-compressed path, we end up just allocating the complete path
without ever checking what the encoded path length actually is. This can
easily lead to a denial of service by just encoding an unreasonable long
path name inside of the index. Git already enforces a maximum path
length of 4096 bytes. As we also have that enforcement ready in some
places, just make sure that the resulting path is smaller than
GIT_PATH_MAX.

Reported-by: Krishna Ram Prakash R <krp@gtux.in>
Reported-by: Vivek Parikh <viv0411.parikh@gmail.com>

The index format in version 4 has prefix-compressed entries, where every
index entry can compress its path by using a path prefix of the previous
entry. Since implmenting support for this index format version in commit
5625d86b (index: support index v4, 2016-05-17), though, we do not
correctly verify that the prefix length that we want to reuse is
actually smaller or equal to the amount of characters than the length of
the previous index entry's path. This can lead to a an integer underflow
and subsequently to an out-of-bounds read.

Fix this by verifying that the prefix is actually smaller than the
previous entry's path length.

Reported-by: Krishna Ram Prakash R <krp@gtux.in>
Reported-by: Vivek Parikh <viv0411.parikh@gmail.com>

The function `read_entry` does not conform to our usual coding style of
returning stuff via the out parameter and to use the return value for
reporting errors. Due to most of our code conforming to that pattern, it
has become quite natural for us to actually return `-1` in case there is
any error, which has also slipped in with commit 5625d86b (index:
support index v4, 2016-05-17). As the function returns an `size_t` only,
though, the return value is wrapped around, causing the caller of
`read_tree` to continue with an invalid index entry. Ultimately, this
can lead to a double-free.

Improve code and fix the bug by converting the function to return the
index entry size via an out parameter and only using the return value to
indicate errors.

Reported-by: Krishna Ram Prakash R <krp@gtux.in>
Reported-by: Vivek Parikh <viv0411.parikh@gmail.com>

deps: upgrade embedded zlib to version 1.2.11

The VM on Travis apparently will still proceed, but it's good practice.

The trusty dependencies are now hosted on Bintray.

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.

The current version of zlib bundled with libgit2 is version 1.2.8. This
version has several CVEs assigned:

- CVE-2016-9843
- CVE-2016-9841
- CVE-2016-9842
- CVE-2016-9840

Upgrade the bundled version to the current release 1.2.11, which has
these vulnerabilities fixes.

adding GIT_FILTER_VERSION to GIT_FILTER_INIT as part of convention

travis: replace use of deprecated homebrew/dupes tap

Test improvements

Introduce a new test suite "odb::backend::simple", which utilizes the
fake backend to exercise the ODB abstraction layer. While such tests
already exist for the case where multiple backends are put together, no
direct testing for functionality with a single backend exist yet.

The fake backend currently implements all reading functions except for
the `exists_prefix` one. Implement it to enable further testing of the
ODB layer.

The `search_object` function takes the OID length as one of its
parameters, where its maximum length is `GIT_OID_HEXSZ`. The `exists`
function of the fake backend used `GIT_OID_RAWSZ` though, leading to
only the first half of the OID being used when finding the correct
object.

In order to be able to test the ODB prefix functions, we need to be able
to detect ambiguous prefixes in case multiple objects with the same
prefix exist in the fake ODB. Extend `search_object` to detect ambiguous
queries and have callers return its error code instead of always
returning `GIT_ENOTFOUND`.

Initialization of the `git_proxy_options` structure is never tested
anywhere. Include it in our usual initialization test in
"core::structinit::compare".

A newly added test uses the `git_repository_new` function without the
corresponding header file being included. While this works due to the
compiler deducing the correct function signature, we should obviously
just include the function's declaration file.

While our debug builds on MSVC platforms already tune the code optimizer
to aid debugging code, all the other platforms still use the default
optimization level. This makes it hard for developers on these platforms
to actually debug code while maintaining his sanity due to optimizations
like inlined code, elided variables etc.

To help this common use case, we can simply follow the MSVC example and
turn off code optimization with "-O0" for debug builds. While it would
be preferable to instead use "-Og" supported by more modern compilers,
we cannot guarantee that this level is available on all supported
platforms.

In our code base, we have some occasions where we use the "_DEBUG"
preprocessor macro to enable additional code which should not be part of
release builds. While we define this flag on MSVC platforms, it is
guarded by the conditional `WIN32 AND NOT CYGWIN` on other platforms
since 19be3f9e (Improve MSVC compiler, linker flags, 2013-02-13). While
this condition can be fulfilled by the MSVC platform, it is never
encountered due to being part of the `ELSE` part of `IF (MSVC)`.

The intention of the conditional was most likely to avoid the
preprocessor macro on Cygwin platforms, but to include it on everthing
else. As such, the correct condition here would be `IF (NOT CYGWIN)`
instead. But digging a bit further, the condition is only ever used in
two places:

1. To skip the test in "core::structinit", which should also work on
   Cygwin.
2. In "src/win32/git2.rc", where it is used to set additional file
   flags. As this file is included in MSVC builds only, it cannot cause
   any harm to set "_DEBUG" on Cygwin here.

As such, we can simply drop the conditional and always set "-D_DEBUG" on
all platforms.