Normalize filediff filenames during creation.

Some of the diff APIs that we're using for post-commit review request creation
(looking at you, GitHub) give us filenames with leading slashes. This doesn't
match what we have elsewhere, which makes things annoying if filenames don't
match.

Testing done:
Used this in conjunction with testing out the suggested reviewers feature. Saw
that when I created a post-commit review request, the filenames matched what
would have been created from rbtools.

Reviewed at https://reviews.reviewboard.org/r/5734/