prune_remote(): sort delete_refs_list references en masse

Inserting items into a list in sorted order is O(N^2) whereas
appending them unsorted and then sorting the list all at once is
O(N lg N).

string_list_insert() also removes duplicates, and this change loses
that functionality. But the strings in this list, which ultimately
come from a for_each_ref() iteration, cannot contain duplicates.

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

builtin/remote.c

@@ -1341,8 +1341,9 @@ static int prune_remote(const char *remote, int dry_run)
 		const char *refname = states.stale.items[i].util;
 
 		delete_refs[i] = refname;
-		string_list_insert(&delete_refs_list, refname);
+		string_list_append(&delete_refs_list, refname);
 	}
+	sort_string_list(&delete_refs_list);
 
 	if (!dry_run) {
 		struct strbuf err = STRBUF_INIT;