fix default branchheads() argument (e85951aeec6e was very broken)

mercurial/localrepo.py

@@ -1205,7 +1205,7 b' class localrepository(repo.repository):'
         return [n for (r, n) in heads]
 
     def branchheads(self, branch=None, start=None):
-        branch = branch is None and 'default' or self.workingctx().branch()
+        branch = branch is None and self.workingctx().branch() or branch
         branches = self.branchtags()
         if branch not in branches:
             return []