"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>