commit: remove dead initialization of 'lock'

The 'lock' variable is initialized to None, but before it's ever read,
it's assigned again.

mercurial/localrepo.py

@@ -1377,7 +1377,7 b' class localrepository(object):'
         Revision information is passed via the context argument.
         """
 
-        tr = lock = None
+        tr = None
         p1, p2 = ctx.p1(), ctx.p2()
         user = ctx.user()