R1:3c785cdb5a9b

include task ids in the commit messages returned by "arc amend"

Summary:
when "arc diff" generates a revision, it attaches a task id
if one is included. However, "arc amend" did not return a task id,
effectively stripping it from the commit message. This diff fixes
that.

NOTE: This is dependent on revision 549 https://secure.phabricator.com/D549

Test Plan:
0. created a custom class to append Facebook task IDs to commit messages and
attached it to the differential.append-commit-message-class config…