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.

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 variable

1. created a new diff in the www repot
2. included Task ID: 609350 in the git commit message
3. "arc diff" to generate the revision
4. "arc amend"
5. ensure that the "Task ID:" field remained in the git commit message

Reviewed By: epriestley
Reviewers: dpepper, jungejason, epriestley
CC: aran, epriestley, mgummelt
Differential Revision: 546