rebase -i: restore autostash on abort

When we abort an interactive rebase we do so by calling
`die_abort`, which cleans up after us by removing the rebase
state directory. If the user has requested to use the autostash
feature, though, the state directory may also contain a reference
to the autostash, which will now be deleted.

Fix the issue by trying to re-apply the autostash in `die_abort`.
This will also handle the case where the autostash does not apply
cleanly anymore by recording it in a user-visible stash.

Reported-by: Daniel Hahler <git@thequod.de>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

git-rebase--interactive.sh

@@ -216,6 +216,7 @@ exit_with_patch () {
 }
 
 die_abort () {
+	apply_autostash
 	rm -rf "$state_dir"
 	die "$1"
 }

t/t3420-rebase-autostash.sh

@@ -192,4 +192,35 @@ test_expect_success 'abort rebase -i with --autostash' '
 	test_cmp expected file0
 '
 
+test_expect_success 'restore autostash on editor failure' '
+	test_when_finished "git reset --hard" &&
+	echo uncommitted-content >file0 &&
+	(
+		test_set_editor "false" &&
+		test_must_fail git rebase -i --autostash HEAD^
+	) &&
+	echo uncommitted-content >expected &&
+	test_cmp expected file0
+'
+
+test_expect_success 'autostash is saved on editor failure with conflict' '
+	test_when_finished "git reset --hard" &&
+	echo uncommitted-content >file0 &&
+	(
+		write_script abort-editor.sh <<-\EOF &&
+			echo conflicting-content >file0
+			exit 1
+		EOF
+		test_set_editor "$(pwd)/abort-editor.sh" &&
+		test_must_fail git rebase -i --autostash HEAD^ &&
+		rm -f abort-editor.sh
+	) &&
+	echo conflicting-content >expected &&
+	test_cmp expected file0 &&
+	git checkout file0 &&
+	git stash pop &&
+	echo uncommitted-content >expected &&
+	test_cmp expected file0
+'
+
 test_done