Allow {} to be used for quoting in query expressions

This makes it a little easier to query for group names that contain
a space over SSH:

  ssh srv gerrit query " 'status:open NOT reviewerin:{Developer Group}' "

Change-Id: I7c81c752236c85d4ed5bd3ed7ad79486b3336510

gerrit-antlr/src/main/antlr3/com/google/gerrit/server/query/Query.g

@@ -155,6 +155,10 @@ EXACT_PHRASE
       String s = $text;
       setText(s.substring(1, s.length() - 1));
     }
+  | '{' ( ~('{'|'}') )* '}' {
+      String s = $text;
+      setText(s.substring(1, s.length() - 1));
+    }
   ;
 
 SINGLE_WORD