stash: add a test for when apply fails during stash branch

If the return value of merge recursive is not checked, the stash could end
up being dropped even though it was not applied properly

Signed-off-by: Joel Teichroeb <joel@teichroeb.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3903-stash.sh

@@ -656,6 +656,20 @@ test_expect_success 'stash branch should not drop the stash if the branch exists
 	git rev-parse stash@{0} --
 '
 
+test_expect_success 'stash branch should not drop the stash if the apply fails' '
+	git stash clear &&
+	git reset HEAD~1 --hard &&
+	echo foo >file &&
+	git add file &&
+	git commit -m initial &&
+	echo bar >file &&
+	git stash &&
+	echo baz >file &&
+	test_when_finished "git checkout master" &&
+	test_must_fail git stash branch new_branch stash@{0} &&
+	git rev-parse stash@{0} --
+'
+
 test_expect_success 'stash apply shows status same as git status (relative to current directory)' '
 	git stash clear &&
 	echo 1 >subdir/subfile1 &&