I created a commit and revision then used `hg amend -e` to modify the commit message locally.
I used `arc amend` to update the commit message back to the appropriate message from Phabricator.
I ran `arc amend` again to verify that the command did not fail even though the commit message is unchanged.
I ran `arc inspect --explore -- 'commit(674492bb460)'` and verified it matched the appropriate commit, to verify the substring matching works properly.