mercurial/dirstate.py

--- a/mercurial/dirstate.py	Thu Feb 11 17:13:22 2010 -0600
+++ b/mercurial/dirstate.py	Thu Feb 11 17:44:01 2010 -0600
@@ -208,6 +208,8 @@
         self._pl = p1, p2
 
     def setbranch(self, branch):
+        if branch in ['tip', '.', 'null']:
+            raise util.Abort(_('the name \'%s\' is reserved') % branch)
         self._branch = branch
         self._opener("branch", "w").write(branch + '\n')