amend: add config option to update time to current in hg amend (issue5828)
The given config option i.e. `rewrite.update-timestamp` updates date to
current when //True//. However when only date is to be updated to current
with the working directory clean and no other attributes changing then it
does not amend as stated in issue 5828. Further when `--date` flag is specified
along with the new config option then `--date` is given priority over the
config option.
Differential Revision: https://phab.mercurial-scm.org/D5491
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