localrepo: use revsymbol in lookupbranch() too

lookupbranch() takes a string that comes from the CLI, so
scmutil.revsymbol() is appropriate for looking it up.

Differential Revision: https://phab.mercurial-scm.org/D3080

mercurial/localrepo.py

@@ -1023,7 +1023,7 b' class localrepository(object):'
         if key in self.branchmap():
             return key
 
-        return self[key].branch()
+        return scmutil.revsymbol(self, key).branch()
 
     def known(self, nodes):
         cl = self.changelog