describe: setup working tree for --dirty

We don't use NEED_WORK_TREE when running the git-describe builtin,
since you should be able to describe a commit even in a bare repository.
However, the --dirty flag does need a working tree. Since we don't call
setup_work_tree(), it uses whatever directory we happen to be in. That's
unlikely to match our index, meaning we'd say "dirty" even when the real
working tree is clean.

We can fix that by calling setup_work_tree() once we know that the user
has asked for --dirty.

The --broken option also needs a working tree. But because its
implementation calls git-diff-index we don‘t have to setup the working
tree in the git-describe process.

Signed-off-by: Sebastian Staudt <koraktor@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/describe.c

@@ -617,6 +617,7 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 			struct argv_array args = ARGV_ARRAY_INIT;
 			int fd, result;
 
+			setup_work_tree();
 			read_cache_preload(NULL);
 			refresh_index(&the_index, REFRESH_QUIET|REFRESH_UNMERGED,
 				      NULL, NULL, NULL);