pkg/search: use the right method for locking the index

Since probably e93e4f3822 we've been using
(*search.Handler).DescribeLocked instead of
(*search.Handler).Describe when serving describe queries, so the index
was never locked :(

I was so focused on debugging the recursions in describeReally that I
missed that elephant in the room.

I still think the Describe and describeReally recursions protected by
dr.wg is too complicated but that's for another CL.

Fixes #750

Change-Id: I376ee0f807df68ad1ef3752f944edc7c9bdb92ee

pkg/search/describe.go

@@ -43,7 +43,7 @@ func (sh *Handler) serveDescribe(rw http.ResponseWriter, req *http.Request) {
 	dr.fromHTTP(req)
 	ctx := context.TODO()
 
-	res, err := sh.DescribeLocked(ctx, &dr)
+	res, err := sh.Describe(ctx, &dr)
 	if err != nil {
 		httputil.ServeJSONError(rw, err)
 		return