Add a unittest to validate bug #3043, where a duplicate empty config header
could cause deletion of a config entry to fail silently. The bug is currently
unresolved and this test will fail.

Rebase fixes

Handle invalid multiline configuration

`git_rebase_init` and `git_rebase_open` should take a
`git_rebase_options` and use it for future rebase operations on
that `rebase` object.

git_rebase_commit should return `GIT_EUNMERGED` when unmerged items
exist in the index, per the documentation.  Test that this is correct.

In `git_rebase_operation_current()`, indicate when a rebase has not
started (with `GIT_REBASE_NO_OPERATION`) rather than conflating that
with the first operation being in-progress.

Don't assume that comment chars are comment chars, they may be (an
attempt to be escaped).  If so, \; is not a valid escape sequence,
complain.

Combine unquoting and multiline detection to avoid ambiguity when
parsing.

Make sure to also update delta->nfiles when merging diffs

ignore: fix negative ignores without wildcards.

Implement git_submodule_set_branch.

Diff fixes

Fix memleak in test/core/mkdir reported by CRTDBG

revwalk: detect when we're out of interesting commits

Handle hide-then-push in the revwalk

Add memory leak detection/reporting using MSVC CRTDBG facility.

In the prior implementation, enabling GIT_DIFF_UPDATE_INDEX would overwrite
entries in the index with the ones generated from scanning the working if the
OID was the same.

Because this OID comparison ignores file modes, this means an file in the
workdir with only an exec bit difference with the one in the index would end
up being overwritten, resulting in the exec bit being loss. There might be
other related bugs but the fix of comparing OIDs and file modes should
address them all.

The variable noid is guaranteed to be zero at this point of the code path.

When walking backwards and marking parents uninteresting, make sure we
detect when the list of commits we have left has run out of
uninteresting commits so we can stop marking commits as
uninteresting. Failing to do so can mean that we walk the whole history
marking everything uninteresting, which eats up time, CPU and IO for
with useless work.

While pre-marking does look for this, we still need to check during the
main traversal as there are setups for which pre-marking does not leave
enough information in the commits. This can happen if we push a commit
and hide its parent.

As it seems it's not quite clear what it means to push a commit, try to
be more explicit about adding a new root and that we may not see this
commit if it is hidden.

Make sure to pack referenced objects for non-branches

If work_dir is not specified, use repo_dir to test if symlink is supported

Fix checking of return value for regcomp.

The regcomp function returns a non-zero value if compilation of
a regular expression fails. In most places we only check for
negative values, but positive values indicate an error, as well.
Fix this tree-wide, fixing a segmentation fault when calling
git_config_iterator_glob_new with an invalid regexp.

When a commit is first set as unintersting and then pushed, we must take
care that we do not put it into the commit list as that makes us return
at least that commit (but maybe more) as we've inserted it into the list
because we have the assumption that we want anything in the commit list.

When we hide a commit which we later push into the revwalk, we do not
handle this well and return commits which we should not.

Fix describe without refs and OID fallbacks

GIT_UNUSED() macro generates code therefore it should be used after
variable declarations.