amend: add boolean to ignore date-only change

I want to add -D/--currentdate option, which should behave slightly different
from the update-timestamp option.

mercurial/cmdutil.py

@@ -2442,10 +2442,12 b' def amend(ui, repo, old, extra, pats, op'
 
         user = opts.get('user') or old.user()
 
+        datemaydiffer = False  # date-only change should be ignored?
         if opts.get('date'):
             date = dateutil.parsedate(opts.get('date'))
         elif ui.configbool('rewrite', 'update-timestamp'):
             date = dateutil.makedate()
+            datemaydiffer = True
         else:
             date = old.date()
 
@@ -2561,14 +2563,12 b' def amend(ui, repo, old, extra, pats, op'
         if ((not changes)
             and newdesc == old.description()
             and user == old.user()
+            and (date == old.date() or datemaydiffer)
             and pureextra == old.extra()):
             # nothing changed. continuing here would create a new node
             # anyway because of the amend_source noise.
             #
             # This not what we expect from amend.
-            if (date == old.date() or
-                (ui.configbool('rewrite', 'update-timestamp') and
-                 not opts.get('date'))):
                 return old.node()
 
         commitphase = None