Mark person that actually runs arc commit as committer instead of the author.

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