Summary:
`arc commit` and `arc mark-committed` would only add comments <author> committed
this revision, since now everyone can run this commands it makes more sense to
show the actual committer instead of the author.
Test Plan:
Commit (or mark committed) not your revision, Phabricator should add <you>
committed this revision comment instead of <author> committed this revision.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, epriestley
Differential Revision: 1067
f1de90d7ef