ref_remove_duplicates(): avoid redundant bisection

The old code called string_list_lookup(), and if that failed called
string_list_insert(), thus doing the bisection search through the
string list twice in the latter code path.

Instead, just call string_list_insert() right away.  If an entry for
that peer reference name already existed, then its util pointer is
always non-NULL.

Of course this doesn't change the fact that the repeated
string_list_insert() calls make the function scale like O(N^2) if the
input reference list is not already approximately sorted.

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

remote.c

@@ -750,13 +750,15 @@ void ref_remove_duplicates(struct ref *ref_map)
 	struct string_list refs = STRING_LIST_INIT_NODUP;
 	struct string_list_item *item = NULL;
 	struct ref *prev = NULL, *next = NULL;
+
 	for (; ref_map; prev = ref_map, ref_map = next) {
 		next = ref_map->next;
 		if (!ref_map->peer_ref)
 			continue;
 
-		item = string_list_lookup(&refs, ref_map->peer_ref->name);
-		if (item) {
+		item = string_list_insert(&refs, ref_map->peer_ref->name);
+		if (item->util) {
+			/* Entry already existed */
 			if (strcmp(((struct ref *)item->util)->name,
 				   ref_map->name))
 				die("%s tracks both %s and %s",
@@ -767,11 +769,9 @@ void ref_remove_duplicates(struct ref *ref_map)
 			free(ref_map->peer_ref);
 			free(ref_map);
 			ref_map = prev; /* skip this; we freed it */
-			continue;
+		} else {
+			item->util = ref_map;
 		}
-
-		item = string_list_insert(&refs, ref_map->peer_ref->name);
-		item->util = ref_map;
 	}
 	string_list_clear(&refs, 0);
 }