maintner: fix screw-up from CL 42615

See comments on https://golang.org/cl/42615

Change-Id: I0d7889319612fac62934c06c3c082893d9370615
Reviewed-on: https://go-review.googlesource.com/42831
Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>

maintner/github.go

@@ -1367,7 +1367,7 @@ func (p *githubRepoPoller) syncLabels(ctx context.Context) error {
 	if changes == 0 {
 		return nil
 	}
-	p.lastUpdate = time.Now()
+	p.c.addMutation(&maintpb.Mutation{Github: mut})
 	return nil
 }