test-read-cache: setup git dir

b1ef400e (setup_git_env: avoid blind fall-back to ".git") made programs
that tried to access a repository without initializing properly die with
a diagnostic message.  One offender is test-read-cache, which is used in
p0002.  Fix it by calling setup_git_directory() before accessing the
index.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-read-cache.c

@@ -5,6 +5,7 @@ int cmd_main(int argc, const char **argv)
 	int i, cnt = 1;
 	if (argc == 2)
 		cnt = strtol(argv[1], NULL, 0);
+	setup_git_directory();
 	for (i = 0; i < cnt; i++) {
 		read_cache();
 		discard_cache();