shortest: remove unnecessary check for revnum in isvalid()

We now always disambiguating against revnums after we've found an
otherwise valid prefix, so we can safely remove the early isrev()
check in isvalid().

Differential Revision: https://phab.mercurial-scm.org/D3501

mercurial/revlog.py

@@ -1524,7 +1524,7 b' class revlog(object):'
                 return True
             if node is None:
                 raise LookupError(node, self.indexfile, _('no node'))
-            return not isrev(prefix)
+            return True
 
         def maybewdir(prefix):
             return all(c == 'f' for c in prefix)