stash: invoke rerere in case of conflict

"stash apply" directly calls a backend merge function which does not
automatically invoke rerere.  This confuses mergetool when leftover
rerere state is left behind from previous merges.

Invoke rerere explicitly when we encounter a conflict during stash
apply.  This turns the test introduced by the previous commit to
succeed.

Signed-off-by: Phil Hord <hordp@cisco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-stash.sh

@@ -469,6 +469,7 @@ apply_stash () {
 	else
 		# Merge conflict; keep the exit status from merge-recursive
 		status=$?
+		git rerere
 		if test -n "$INDEX_OPTION"
 		then
 			gettextln "Index was not unstashed." >&2