stash: invoke rerere in case of conflict

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>