Make sure git-stash works from subdirectory.

We say "SUBDIRECTORY_OK" but we did not chdir to toplevel; this
is fine as long as everything we use can be started from a
subdirectory, but unfortunately "merge-recursive" is not one of
the programs you can safely use from a subdirectory.

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