Don't parse separator tabs as part of the filename in Git diffs which affect files with spaces

Summary:
Fixes T5870. When a filename contains spaces, Git will add a "\t" at the end of the "---" and "+++" lines. We currently consume this and think we're reading a file called "blah blah\t".

Instead, parse it out as not part of the filename.

Test Plan: Added failing unit test and made it pass.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5870

Differential Revision: https://secure.phabricator.com/D10267
Patch: don't complain about reviewers not being specified.
This will break in the future when upstream arc incorporates T4631. Hopefully there will be a better solution by then