context: fix typo in workingcommitctx

This was probably a copy pasta error in 745e3b485632. Refactoring memctx
code exposed this bug.

mercurial/context.py

@@ -2132,7 +2132,7 b' class workingcommitctx(workingctx):'
     """
     def __init__(self, repo, changes,
                  text="", user=None, date=None, extra=None):
-        super(workingctx, self).__init__(repo, text, user, date, extra,
+        super(workingcommitctx, self).__init__(repo, text, user, date, extra,
                                          changes)
 
     def _dirstatestatus(self, match, ignored=False, clean=False, unknown=False):