Move PatchSet creation into ChangeInserter

This also fixes some probably-unnoticeable inconsistencies in
commit/change/patch set timestamps. Some callers generated new
timestamps for the patch set, and some didn't. Also, the calls to
ChangeUtil.updated(change) before the change was written had been
overriding the manually-set timestamp.

Change-Id: I76bbf9cbed0fd921cef3a2c2847fb03856c6bcb7
4 files changed