midx.c: extract MIDX lookup by object_dir

The first thing that write_midx_internal() does is load the MIDX
corresponding to the given object directory, if one is present.

Prepare for other functions in midx.c to do the same thing by extracting
that operation out to a small helper function.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.c

@@ -1107,6 +1107,22 @@ cleanup:
 	return ret;
 }
 
+static struct multi_pack_index *lookup_multi_pack_index(struct repository *r,
+							const char *object_dir)
+{
+	struct multi_pack_index *cur;
+
+	/* Ensure the given object_dir is local, or a known alternate. */
+	find_odb(r, object_dir);
+
+	for (cur = get_multi_pack_index(r); cur; cur = cur->next) {
+		if (!strcmp(object_dir, cur->object_dir))
+			return cur;
+	}
+
+	return NULL;
+}
+
 static int write_midx_internal(const char *object_dir,
 			       struct string_list *packs_to_include,
 			       struct string_list *packs_to_drop,
@@ -1120,15 +1136,11 @@ static int write_midx_internal(const char *object_dir,
 	struct hashfile *f = NULL;
 	struct lock_file lk;
 	struct write_midx_context ctx = { 0 };
-	struct multi_pack_index *cur;
 	int pack_name_concat_len = 0;
 	int dropped_packs = 0;
 	int result = 0;
 	struct chunkfile *cf;
 
-	/* Ensure the given object_dir is local, or a known alternate. */
-	find_odb(the_repository, object_dir);
-
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name))
 		die_errno(_("unable to create leading directories of %s"),
@@ -1140,12 +1152,7 @@ static int write_midx_internal(const char *object_dir,
 		 * packs to include, since all packs and objects are copied
 		 * blindly from an existing MIDX if one is present.
 		 */
-		for (cur = get_multi_pack_index(the_repository); cur; cur = cur->next) {
-			if (!strcmp(object_dir, cur->object_dir)) {
-				ctx.m = cur;
-				break;
-			}
-		}
+		ctx.m = lookup_multi_pack_index(the_repository, object_dir);
 	}
 
 	if (ctx.m && !midx_checksum_valid(ctx.m)) {