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

t/t7610-mergetool.sh

@@ -203,7 +203,7 @@ test_expect_success 'mergetool skips resolved paths when rerere is active' '
     git reset --hard
 '
 
-test_expect_failure 'conflicted stash sets up rerere'  '
+test_expect_success 'conflicted stash sets up rerere'  '
     git config rerere.enabled true &&
     git checkout stash1 &&
     echo "Conflicting stash content" >file11 &&