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>

git-stash.sh

@@ -6,6 +6,7 @@ USAGE='[ | list | show | apply | clear]'
 SUBDIRECTORY_OK=Yes
 . git-sh-setup
 require_work_tree
+cd_to_toplevel
 
 TMP="$GIT_DIR/.git-stash.$$"
 trap 'rm -f "$TMP-*"' 0