amend: turn currentdate into tri-state bool so config option can be negated

Otherwise, rewrite.update-timestap couldn't be overridden by --no-currentdate.

FWIW, which do we like? --no-currentdate vs --no-current-date. If the latter
is preferred, maybe we can add "graft --current-date" and deprecate
"--currentdate".

hgext/amend.py

@@ -36,7 +36,7 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,
+     ('D', 'currentdate', None,
       _('record the current date as commit date')),
     ] + cmdutil.walkopts + cmdutil.commitopts + cmdutil.commitopts2,
     _('[OPTION]... [FILE]...'),

mercurial/cmdutil.py

@@ -2450,7 +2450,8 b' def amend(ui, repo, old, extra, pats, op'
             date = dateutil.parsedate(opts.get('date'))
         elif opts.get('currentdate'):
             date = dateutil.makedate()
-        elif ui.configbool('rewrite', 'update-timestamp'):
+        elif (ui.configbool('rewrite', 'update-timestamp')
+              and opts.get('currentdate') is None):
             date = dateutil.makedate()
             datemaydiffer = True
         else:

tests/test-amend.t

@@ -453,6 +453,14 b' Unlike rewrite.update-timestamp, -D/--cu'
   date:        Thu Jan 01 00:00:05 1970 +0000
   summary:     commit 1
 
+rewrite.update-timestamp can be negated by --no-currentdate
+
+  $ hg amend --config rewrite.update-timestamp=True --no-currentdate -u baz
+  $ hg log --limit 1
+  user:        baz
+  date:        Thu Jan 01 00:00:05 1970 +0000
+  summary:     commit 1
+
 Bad combination of date options:
 
   $ hg amend -D --date '0 0'