R1:e46e383bf25f

Clean up "Revisions/Commits" table in Maniphest slightly

Summary: Ref T13440. Give the table more obvious visual structure and get rid of the largely useless header columns.

Test Plan: Viewed table, saw a slightly cleaner result.

Maniphest Tasks: T13440

Differential Revision: https://secure.phabricator.com/D20885