revset: fix parameter name in implementation of follow() stable

revset: fix parameter name in implementation of follow()

--- a/mercurial/revset.py	Mon Jun 20 22:15:52 2011 -0500
+++ b/mercurial/revset.py	Fri Jun 24 00:18:06 2011 +0200
@@ -425,7 +425,7 @@
     s |= set([p])
     return [r for r in subset if r in s]
 
-def followfile(repo, subset, f):
+def followfile(repo, subset, x):
     """``follow()``
     An alias for ``::.`` (ancestors of the working copy's first parent).
     """