builtin-prune: protect objects listed on the command line

Finally, this resurrects the documented behaviour to protect other
objects listed on the command line from getting pruned.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-prune.c

@@ -140,8 +140,19 @@ int cmd_prune(int argc, const char **argv, const char *prefix)
 	init_revisions(&revs, prefix);
 
 	argc = parse_options(argc, argv, options, prune_usage, 0);
-	if (argc)
+	while (argc--) {
-		die ("unrecognized argument: %s", name);
+		unsigned char sha1[20];
+		const char *name = *argv++;
+
+		if (!get_sha1(name, sha1)) {
+			struct object *object = parse_object(sha1);
+			if (!object)
+				die("bad object: %s", name);
+			add_pending_object(&revs, object, "");
+		}
+		else
+			die("unrecognized argument: %s", name);
+	}
 	mark_reachable_objects(&revs, 1);
 	prune_object_dir(get_object_directory());
 

t/t5304-prune.sh

@@ -100,7 +100,7 @@ test_expect_success 'prune: prune unreachable heads' '
 
 '
 
-test_expect_failure 'prune: do not prune heads listed as an argument' '
+test_expect_success 'prune: do not prune heads listed as an argument' '
 
 	: > file2 &&
 	git add file2 &&