Since I don't have permission yet on the code from Git, I decided
I'd take a stab at writing patterns for PHP and Javascript myself.
I think these are pretty weak, but probably better than the
default behavior without them.

This moves the expected and actual test data along with the source
data for the userdiff tests into the tests/resources/userdiff test
repo and updates the test to use that.

This brings over the Pascal and CSharp userdiff data.

I contacted a number of Git authors and lined up their permission
to relicense their work for use in libgit2 and copied over their
code for diff driver xfuncname patterns.  At this point, the code
I've copied is taken verbatim from core Git although Thomas Rast
warned me that the C++ patterns, at least, really need an update.
I've left off patterns where I don't feel like I have permission
at this point until I hear from more authors.

Reorganize the builtin driver table slightly so that core Git
builtin definitions can be imported verbatim.  Then take a few of
the core Git drivers and pull them in.

This also creates a test of diffs with the builtin HTML driver
which led to some small error handling fixes in the driver
selection logic.

This extends the diff driver parser to support multiline driver
definitions along with ! prefixing for negated matches.  This
brings the driver function pattern parsing in line with core Git.

This also adds an internal table of driver definitions and a
fallback code path that will look in that table for diff drivers
that are set with attributes without having a definition in the
config file.  Right now, I just populated the table with a kind
of simple HTML definition that is similar to the core Git def.

Drop parsing pack filename SHA1 part

Sometimes a zero byte file is just a zero byte file

Don't go to the ODB to resolve zero byte files in the workdir

Summarize empty messages

Preserve tree filemode in index during checkout

Don't try to determine whether the system supports file modes
when putting the tree data in the index during checkout.  The tree's
mode is canonical and did not come from stat(2) in the first place.

Expose patch serialization to git_buf

It's hard or even impossible to correctly free the string buffer
allocated by git_patch_to_str in some circumstances. Drop the function
so people have to use git_patch_to_buf instead - git_buf has a dedicated
destructor.

Returning library-allocated strings from libgit2 works fine on Linux,
but may cause problems on Windows because there is no one C Runtime that
everything links against. With libgit2 not exposing its own allocator,
freeing the string is a gamble.

git_patch_to_str already serializes to a buffer, then returns the
underlying memory. Expose the functionality directly, so callers can use
the git_buf_free function to free the memory later.

Merge improvements

The "merge none" (don't automerge) flag was only to aide in
merge trivial tests.  We can easily determine whether merge
trivial resulted in a trivial merge or an automerge by examining
the REUC after automerge has completed.

The default merge_file level was XDL_MERGE_MINIMAL, which will
produce conflicts where there should not be in the case where
both sides were changed identically.  Change the defaults to be
more aggressive (XDL_MERGE_ZEALOUS) which will more aggressively
compress non-conflicts.  This matches git.git's defaults.

Increase testing around reverting a previously reverted commit to
illustrate this problem.

"Uninitialized" submodules are "unmodified"

Extend the "unmodified" submodule workdir test to include
uninitialized submodules, to prevent reporting submodules as
modified when they're not in the workdir at all.

git_reflog_entry_message can be null

Fix a couple of corner cases and an undefined behavior

Fix segfault when calling git_config_get_* functions when a config fails to load

Reinitialize the result code of get_entry() to GIT_ENOTFOUND

Convert gitdir paths to posix on Windows

Apparently, a .git file with "gitdir: path" link on Windows is
allowed to use backslashes in the path.  Who knew?