If given a NULL path, write to the object path of the repository.

Add tests for the new behavior.

Clean up and return via a single label, to avoid duplicate error
handling before each return, and to make it easier to extend the set of
cleanups needed.

OpenSSL certificate memory leak

tests: checkout: fix flaky test due to mtime race

cmake: Sort source files for reproducible builds

When trying to determine whether a file changed, we try to avoid heavy
operations by fist taking a look at the index, seeing whether the index
entry is modified already. This doesn't seem to cut it, though, as we
currently have the racy checkout::index::can_disable_pathspec_match test
case: sometimes the files get restored to their original contents,
sometimes they aren't.

The issue is caused by a racy index [1]: in case we modify a file, add
it to the index and then modify it again in-place without changing its
file, then we may end up with a modified file that has the same stat(3P)
info as we've currently got it in its corresponding index entry. The
mitigation for this is to treat files with the same mtime as the index
are treated as racily modified. We already have this logic in place for
the index, but not when doing a checkout.

Fix the issue by only consulting the index entry in case it has an older
mtime as the index. Previously, the following script reliably had at
least 20 failures, while now there is no failure to be observed anymore:

```bash
j=0
for i in $(seq 100)
do
	if ! ./libgit2_clar -scheckout::index::can_disable_pathspec_match >/dev/null
	then
		j=$(($j + 1))
	fi
done
echo "Failures: $j"
```

[1]: https://git-scm.com/docs/racy-git

The test case checkout::index::can_disable_pathspec_match has some
shortcomings when it comes to coding style, which didn't fit our own
coding style. Furthermore, it had an unnecessary static local variable.

The test has been refactored to address these issues.

We currently use `FILE(GLOB ...)` in most places to find source and
header files. This is problematic in that the order of files returned
depends on the operating system's directory iteration order and may thus
not be deterministic. As a result, we link object files in unspecified
order, which may cause the linker to emit different code across runs.

Fix this issue by sorting all code used as input to the libgit2 library
to improve the reliability of reproducible builds.

When creating a `git_cert` from the OpenSSL X509 certificate of a given
stream, we do not call `X509_free()` on the certificate, leading to a
memory leak as soon as the certificate is requested e.g. by the
certificate check callback.

Fix the issue by properly calling `X509_free()`.

futils: fix order of declared parameters for `git_futils_fake_symlink`

While the function `git_futils_fake_symlink` is declared with arguments
`new, old`, the implementation uses the reverse order `old, new`. Let's
fix the ordering issues to be `new, old` for both, which matches what
symlink(3P) has. While at it, we also rename these parameters: `old` and
`new` doesn't really make a lot of sense in the context of symlinks,
which is why this commit renames them to be called `target` and `path`.

Check the version in package.json

tests: merge: fix printf formatter on 32 bit arches

We currently use `PRIuMAX` to print an integer of type `size_t` in
merge::trees::rename::cache_recomputation. While this works just fine on
64 bit arches, it doesn't on 32 bit ones. As a result, our nightly
builds on x86 and arm32 fail.

Fix the issue by using `PRIuZ` instead.

Update package.json

Introduce GIT_ASSERT macros

README.md: Add instructions for building in MinGW environment

Include GIT_ASSERT_WITH_RETVAL and GIT_ASSERT_ARG_WITH_RETVAL so that
functions that do not return int (or more precisely, where `-1` would
not be an error code) can assert.

This allows functions that return, eg, NULL on an error code to do that
by passing the return value (in this example, `NULL`) as a second
parameter to the GIT_ASSERT_WITH_RETVAL functions.

Fall back to the system assert(3) in debug builds, which may aide
in debugging.

"Safe" assertions can be enabled in debug builds by setting
GIT_ASSERT_HARD=0.  Similarly, hard assertions can be enabled in
release builds by setting GIT_ASSERT_HARD to nonzero.

Provide macros to replace usages of `assert`.  A true `assert` is
punishing as a library.  Instead we should do our best to not crash.

GIT_ASSERT_ARG(x) will now assert that the given argument complies to
some format and sets an error message and returns `-1` if it does not.

GIT_ASSERT(x) is for internal usage, and available as an internal
consistency check.  It will set an error message and return `-1` in the
event of failure.

Add instructions for building libgit2 in MinGW environment

Fix uninitialized stack memory and NULL ptr dereference in stash_to_index

Caught by static analysis.

The checkout code didn't iterate into a subdir if it didn't match the
pathspec, but since the pathspec might match files in the subdir we
should recurse into it (In contrast to gitignore handling).

Fixes #5089

Honor GIT_CHECKOUT_DISABLE_PATHSPEC_MATCH for all checkout types 

We were previously applying the pathspec filter for the baseline
iterator during checkout, as well as the target tree.  This was an
oversight; in fact, we should apply the pathspec filter to _all_
checkout targets, not just trees.

Add a helper function to set the iterator pathspecs from the given
checkout pathspecs, and call it everywhere.

The checkout::index::can_disable_pathspec_match test attempts to set a
path filter of `test11.txt` and `test12.txt`, but then validates that
`test10.txt` and `test11.txt` were left unmodified.  Update the test's
path filter to match the expectation.

docs: add documentation for our coding style

MSVC: Enable Control Flow Guard (CFG)

git__hexdump: better mimic `hexdump -C`

This feature requires Visual Studio 2015 (MSVC_VERSION = 1900) or later. As the
minimum required CMake version is currently less than 3.7, GREATER_EQUAL is not
available to us and we must invert the result of the LESS operator.

Feature: Allow blame to ignore whitespace change

deps: ntlmclient: use htobe64 on NetBSD too

For years, we've repeatedly had confusion about what our actual coding
style is not only for newcomers, but also across the core contributors.
This can mostly be attributed to the fact that we do not have any coding
conventions written down. This is now a thing of the past with the
introduction of a new document that gives an initial overview of our
style and most important best practices for both our C codebase as well
as for CMake.

While the proposed coding style for our C codebase should be rather
uncontroversial, the coding style for CMake might be. This can be
attributed to multiple facts. First, the CMake code base doesn't really
have any uniform coding style and is quite outdated in a lot of places.
Second, the proposed coding style actually breaks with our existing one:
we currently use all-uppercase function names and variables, but the
documented coding style says we use all-lowercase function names but
all-uppercase variables.

It's common practice in CMake to write variables in all upper-case, and
in fact all variables made available by CMake are exactly that. As
variables are case-sensitive in CMake, we cannot and shouldn't break
with this. In contrast, function calls are case insensitive, and modern
CMake always uses all-lowercase ones. I argue we should do the same to
get in line with other codebases and to reduce the likelihood of
repetitive strain injuries.

So especially for CMake, the proposed coding style says something we
don't have yet. I'm fine with that, as the document explicitly says that
it's what we want to have and not what we have right now.