Users cannot attach diffs to revisions they don't own anymore.

Summary:
Users were able to accidentally update revisions they didn't own. Now
it is impossible to update a revision that belongs to someone else or
has been marked as committed.

Test Plan:
Tested that normal workflow works as previously, but after running
'arc amend', running 'arc diff' fails.

Manually changed the revision number in the git commit message and tried
to update something that belongs to Jason -> Failed.

Reviewed By: epriestley
Reviewers: epriestley
CC: jungejason, epriestley, tuomaspelkonen
Differential Revision: 112