This makes no functional change to diff but renames a couple of
the objects and splits the new git_patch (formerly git_diff_patch)
into a new header file.
There were a lot of places in the test code base that were creating
a commit from the index on the current branch. This just adds a
helper to handle that case pretty easily. There was only one test
where this change ended up tweaking the test data, so pretty easy
and mostly just a cleanup.
Create a new section of clar tests "stress" that will default to
being off where we can put slow tests that push the library for
performance testing purposes.
