Fix repo.index.diff("HEAD", create_patch=True) always returning an empty list

[thetwoj]

This PR resolves the issue reported in #852

As @jadinm correctly identified in #852, this bug centers around the fact that the regex utilized by _index_from_patch_format() in git/diff.py always expects the first line of the diff to have the a file first, then b:

diff --git a/the_file.py b/the_file.py

When the other passed to repo.index.diff(other) is not None the -R argument is added to the git command, resulting in the positions of the a and b files being flipped in the output and causing the _index_from_patch_format() function invoked by the create_patch option to not find a regex match:

diff --git b/the_file.py a/the_file.py

This fix makes the regex slightly more permissive, allowing it to match either order of the a and b files in the output of git diff.

For test coverage of this change I added assertions to an existing test within test_diff.py that set up everything needed to reproduce this bug.

Massive props to @jadinm, their troubleshooting and feedback on the issue significantly shortened the investigation required to get this one sorted out 🎉

[Byron]

I am absolutely amazed by your contribution, and truly value the time and energy you must put into them!

Please let me know if you can think of anything that I can do to make your life as a contributor easier.

[thetwoj]

Thanks @Byron! I appreciate your willingness to entertain these PRs and how promptly you review them, it makes contributing fun 👍