cmdutil: fix amend when passing a date

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.

Differential Revision: https://phab.mercurial-scm.org/D691

mercurial/cmdutil.py

@@ -3059,6 +3059,9 b' def amend(ui, repo, old, extra, pats, op'
         user = opts.get('user') or old.user()
         date = opts.get('date') or old.date()
 
+        # Parse the date to allow comparison between date and old.date()
+        date = util.parsedate(date)
+
         if len(old.parents()) > 1:
             # ctx.files() isn't reliable for merges, so fall back to the
             # slower repo.status() method

tests/test-amend.t

@@ -85,6 +85,14 b' Nothing changed'
   nothing changed
   [1]
 
+  $ hg amend -d "0 0"
+  nothing changed
+  [1]
+
+  $ hg amend -d "Thu Jan 01 00:00:00 1970 UTC"
+  nothing changed
+  [1]
+
 Matcher and metadata options
 
   $ echo 3 > C