Following https://phab.mercurial-scm.org/D636, passing the same date that the
changeset to amend would results in no new commits but the output changed
from:
$ hg amend -d '0 0' nothing changed [1]
to:
$ hg amend -d '0 0'
Restore the old behavior by parsing the date passed as parameter so the
condition "date == old.date()" correctly works in cases both dates are
identical.
Add a test for covering this regression.
This bug was found thanks to Evolve test suite.