git-stash: Make sure reflog is created for refs/stash

Earlier commit 7ab3cc70 fixed "stash clear" but broke save_stash,
because it forgot to make sure the reflog file exists before saving.

Signed-off-by: Nanako Shiraishi <nanako3@bluebottle.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-stash.sh

@@ -36,6 +36,9 @@ save_stash () {
 	test -f "$GIT_DIR/logs/$ref_stash" ||
 		clear_stash || die "Cannot initialize stash"
 
+	# Make sure the reflog for stash is kept.
+	: >>"$GIT_DIR/logs/$ref_stash"
+
 	# state of the base commit
 	if b_commit=$(git rev-parse --verify HEAD)
 	then