histedit: add rewrite.update-timestamp support to fold and mess This adds the config option to update time to current in histedit fold and mess options. Setting rewrite.update-timestamp option to `True` will update the timestamp to current time. This also adds `tests/mockmakedate.py` for supplying testable values in case current time is invoked in the tests. Differential Revision: https://phab.mercurial-scm.org/D5554