upload-pack: move "unshallow" sending code out of deepen()

Also add some more comments in this code because it takes too long to
understand what it does (to me, who should be familiar enough to
understand this code well!)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

upload-pack.c

@@ -552,20 +552,10 @@ static void send_shallow(struct commit_list *result)
 	}
 }
 
-static void deepen(int depth, const struct object_array *shallows)
+static void send_unshallow(const struct object_array *shallows)
 {
-	struct commit_list *result = NULL;
 	int i;
-	if (depth == INFINITE_DEPTH && !is_repository_shallow())
-		for (i = 0; i < shallows->nr; i++) {
-			struct object *object = shallows->objects[i].item;
-			object->flags |= NOT_SHALLOW;
-		}
-	else
-		result = get_shallow_commits(&want_obj, depth,
-					     SHALLOW, NOT_SHALLOW);
-	send_shallow(result);
-	free_commit_list(result);
+
 	for (i = 0; i < shallows->nr; i++) {
 		struct object *object = shallows->objects[i].item;
 		if (object->flags & NOT_SHALLOW) {
@@ -573,7 +563,13 @@ static void deepen(int depth, const struct object_array *shallows)
 			packet_write(1, "unshallow %s",
 				     oid_to_hex(&object->oid));
 			object->flags &= ~CLIENT_SHALLOW;
-			/* make sure the real parents are parsed */
+			/*
+			 * We want to _register_ "object" as shallow, but we
+			 * also need to traverse object's parents to deepen a
+			 * shallow clone. Unregister it for now so we can
+			 * parse and add the parents to the want list, then
+			 * re-register it.
+			 */
 			unregister_shallow(object->oid.hash);
 			object->parsed = 0;
 			parse_commit_or_die((struct commit *)object);
@@ -588,6 +584,27 @@ static void deepen(int depth, const struct object_array *shallows)
 		/* make sure commit traversal conforms to client */
 		register_shallow(object->oid.hash);
 	}
+}
+
+static void deepen(int depth, const struct object_array *shallows)
+{
+	if (depth == INFINITE_DEPTH && !is_repository_shallow()) {
+		int i;
+
+		for (i = 0; i < shallows->nr; i++) {
+			struct object *object = shallows->objects[i].item;
+			object->flags |= NOT_SHALLOW;
+		}
+	} else {
+		struct commit_list *result;
+
+		result = get_shallow_commits(&want_obj, depth,
+					     SHALLOW, NOT_SHALLOW);
+		send_shallow(result);
+		free_commit_list(result);
+	}
+
+	send_unshallow(shallows);
 	packet_flush(1);
 }