revset: make optimize() reject unknown operators

This should have caught the bug of 'keyvalue' operator fixed at 5004ef47f437.
The catch-all pattern is useless since optimize() should be aware of all known
operators.

mercurial/revset.py

@@ -2463,7 +2463,7 b' def _optimize(x, small):'
         else:
             w = 1
         return w + wa, (op, x[1], ta)
-    return 1, x
+    raise ValueError('invalid operator %r' % op)
 
 def optimize(tree):
     _weight, newtree = _optimize(tree, small=True)