query: Simplify empty branch and repo patterns

For example this can happen when using the REST API, where the list of
restrictions is converted into a query. For large restriction lists this can
greatly simplify the final query passed to Searcher.

Change-Id: I8d3bb60f5943975501ac5bbb1ff097f0452a3ab9

query/query.go

@@ -281,6 +281,14 @@ func evalConstants(q Q) Q {
 		if s.Regexp.Op == syntax.OpEmptyMatch {
 			return &Const{true}
 		}
+	case *Branch:
+		if s.Pattern == "" {
+			return &Const{true}
+		}
+	case *Repo:
+		if s.Pattern == "" {
+			return &Const{true}
+		}
 	}
 	return q
 }