Make git diff command lines more sensible.

When posting a git change against a server which has the renames capability,
the diff command line would include both `--no-renames` and `-M`. Git seems to
prefer `-M`, but it's kind of nonsense to have both. This change makes it an
either/or deal.

Testing done: Smoke tested git posting.

Reviewed at https://reviews.reviewboard.org/r/5975/