test-*-cache-tree: setup git dir

These test helper programs access the index, but do not ever
setup_git_directory(), meaning we just blindly looked in
".git/index". This happened to work for the purposes of our
tests (which do not run from subdirectories, nor in
non-repos), but it's a bad habit.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-dump-cache-tree.c

@@ -58,6 +58,7 @@ int cmd_main(int ac, const char **av)
 {
 	struct index_state istate;
 	struct cache_tree *another = cache_tree();
+	setup_git_directory();
 	if (read_cache() < 0)
 		die("unable to read index file");
 	istate = the_index;