Simplify "Blame Revision" field in Differential

Summary: Ref T11114. This is still mostly in use, but toss a few commit message parsing things.

Test Plan: Viewed/edited/upated blame rev from CLI/web UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17089