Fix commit date (issue1193)

The None check in workingctx prevents calling util.makedate
when date is not explicitly given.

mercurial/context.py

@@ -458,7 +458,7 b' class workingctx(changectx):'
         self._rev = None
         self._node = None
         self._text = text
-        if date is not None:
+        if date:
             self._date = util.parsedate(date)
         else:
             self._date = util.makedate()