Clean up the git diff call a little bit

In particular -M (--find-renames) and --no-renames should be mutually exclusive
and the revision range should come last in the diff command before any options
terminating dashes (--) and paths. git doesn't enforce this today, but it could
tomorrow.

Testing done: Posted a review request with rbt.

Reviewed at https://reviews.reviewboard.org/r/4823/