Remove `commit` from DiffusionRequest

Summary: Ref T2683. This field is //almost// entirely redundant with `symbolicCommit`. Improve how some of the diff query stuff works a bit, then remove it.

Test Plan: Browsed around in all interfaces, looked at a bunch of diffs, etc.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9099