prune_refs(): use delete_refs()

The old version just looped over the references to delete, calling
delete_ref() on each one. But that has quadratic behavior, because
each call to delete_ref() might have to rewrite the packed-refs file.
This can be very expensive in a repository with a large number of
references. In some (admittedly extreme) repositories, we've seen
cases where the ref-pruning part of fetch takes multiple tens of
minutes.

Instead call delete_refs(), which (aside from being less code) has the
optimization that it only rewrites the packed-refs file a single time.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -790,20 +790,29 @@ static int prune_refs(struct refspec *refs, int ref_count, struct ref *ref_map,
 	if (4 < i && !strncmp(".git", url + i - 3, 4))
 		url_len = i - 3;
 
+	if (!dry_run) {
+		struct string_list refnames = STRING_LIST_INIT_NODUP;
+
+		for (ref = stale_refs; ref; ref = ref->next)
+			string_list_append(&refnames, ref->name);
+
+		result = delete_refs(&refnames);
+		string_list_clear(&refnames, 0);
+	}
+
+	if (verbosity >= 0) {
 		for (ref = stale_refs; ref; ref = ref->next) {
-		if (!dry_run)
+			if (!shown_url) {
-			result |= delete_ref(ref->name, NULL, 0);
-		if (verbosity >= 0 && !shown_url) {
 				fprintf(stderr, _("From %.*s\n"), url_len, url);
 				shown_url = 1;
 			}
-		if (verbosity >= 0) {
 			fprintf(stderr, " x %-*s %-*s -> %s\n",
 				TRANSPORT_SUMMARY(_("[deleted]")),
 				REFCOL_WIDTH, _("(none)"), prettify_refname(ref->name));
 			warn_dangling_symref(stderr, dangling_msg, ref->name);
 		}
 	}
+
 	free(url);
 	free_refs(stale_refs);
 	return result;