localrepo: use revsymbol() in lookup()

lookup() seems to be about looking up a revision based on a symbol
that may come from the user (via the wire protocol), so revsymbol() is
appropriate here.

Differential Revision: https://phab.mercurial-scm.org/D3055

mercurial/localrepo.py

@@ -1021,7 +1021,7 b' class localrepository(object):'
                 pass
 
     def lookup(self, key):
-        return self[key].node()
+        return scmutil.revsymbol(self, key).node()
 
     def lookupbranch(self, key, remote=None):
         repo = remote or self