Add `git_commit_descendant_of`.

Add built in diff drivers

Writing a sample Javascript driver pointed out some extra
whitespace handling that needed to be done in the diff driver.
This adds some tests with some sample javascript code that I
pulled off of GitHub just to see what would happen.  Also, to
clean up the userdiff test data, I did a "git gc" and packed
up the test objects.

Leftover OID -> ID changes

Buff up returning strings

MSVC doesn't like modern code neither

Make blame example compile on MSVC

A lot of the tests were checking for overflow, which we don't have
anymore, so we can remove them.

Again, we already did this internally, so simply remove the conversions.

Internally we already did everything with git_bufs, so this is just
exposing those functions with public names.

Since we now export that type, we can avoid making the user guess a
size.

Fix git_remote_dup & add missing const

Fix write_object.

Fix local push to file:// URL.

Following the rest of the series, use 'id' when refering to the value.

Same as the other commits in the series, we use 'id' when talking about
thing rather than the datatype.

In the same vein as the previous commits in this series.

Rename git_tree_entry_byoid() to _byid() as per the convention.

This was not converted when we converted the rest, so do it now.

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.

This was left over when we did the general switch.