amend: add -D/--currentdate option

It bumps the date field even if there's no other change. The help text is
copied from commands.graft().

Unlike graft, --date with --currentdate is disallowed, which I think is
saner behavior.

hgext/amend.py

@@ -36,6 +36,8 b' command = registrar.command(cmdtable)'
      ('e', 'edit', None, _('invoke editor on commit messages')),
      ('i', 'interactive', None, _('use interactive mode')),
      ('n', 'note', '', _('store a note on the amend')),
+     ('D', 'currentdate', False,
+      _('record the current date as commit date')),
     ] + cmdutil.walkopts + cmdutil.commitopts + cmdutil.commitopts2,
     _('[OPTION]... [FILE]...'),
     helpcategory=command.CATEGORY_COMMITTING,

mercurial/cmdutil.py

@@ -2443,8 +2443,13 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') and opts.get('currentdate'):
+            raise error.Abort(_('--date and --currentdate are mutually '
+                                'exclusive'))
         if opts.get('date'):
             date = dateutil.parsedate(opts.get('date'))
+        elif opts.get('currentdate'):
+            date = dateutil.makedate()
         elif ui.configbool('rewrite', 'update-timestamp'):
             date = dateutil.makedate()
             datemaydiffer = True

tests/test-amend.t

@@ -438,3 +438,25 b' When date option is applicable and updat'
   user:        foobar
   date:        Thu Jan 01 00:01:00 1998 +0000
   summary:     commit 1
+
+Unlike rewrite.update-timestamp, -D/--currentdate always updates the timestamp
+
+  $ hg amend -D
+  $ hg log --limit 1
+  user:        foobar
+  date:        Thu Jan 01 00:00:04 1970 +0000
+  summary:     commit 1
+
+  $ hg amend -D --config rewrite.update-timestamp=True
+  $ hg log --limit 1
+  user:        foobar
+  date:        Thu Jan 01 00:00:05 1970 +0000
+  summary:     commit 1
+
+Bad combination of date options:
+
+  $ hg amend -D --date '0 0'
+  abort: --date and --currentdate are mutually exclusive
+  [255]
+
+  $ cd ..