R1:e9bd84222701

Allow "blame previous revision" to track file moves and handle edge cases

Summary:
- Track + message through file moves.
- Stop + message on file create.
- Stop + message on first commit.

Test Plan:
- Tested blaming through a move, through a create, and through the first commit.
- Verified this doesn't break anything in SVN / Mercurial.

Reviewers: vrana, btrahan, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1091

Differential Revision: https://secure.phabricator.com/D2295