Search: adjust association of negation

Make negation left-associative, so that it groups from the left.
This seems to match what Gerrit does.  This means that:

  not owner:self project:foo

is:

  (not owner:self) and (project:foo)

rather than the previous, erroneous behavior:

  not (owner:self and project:foo)

Change-Id: I9a922fb16d0b0e0c3d17871dc0e74f2211e36399

gertty/search/parser.py

@@ -24,6 +24,10 @@ import gertty.search
 from tokenizer import tokens
 
 def SearchParser():
+    precedence = (
+        ('left', 'NOT', 'NEG'),
+    )
+
     def p_terms(p):
         '''expression : list_expr
                       | paren_expr