rebase: do not attempt to remove startup_info->original_cwd

Since rebase spawns a `checkout` subprocess, make sure we run that from
the startup_info->original_cwd directory, so that the checkout process
knows to protect that directory.

Acked-by: Derrick Stolee <stolee@gmail.com>
Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -4228,6 +4228,8 @@ static int run_git_checkout(struct repository *r, struct replay_opts *opts,
 
 	cmd.git_cmd = 1;
 
+	if (startup_info->original_cwd)
+		cmd.dir = startup_info->original_cwd;
 	strvec_push(&cmd.args, "checkout");
 	strvec_push(&cmd.args, commit);
 	strvec_pushf(&cmd.env_array, GIT_REFLOG_ACTION "=%s", action);

t/t2501-cwd-empty.sh

@@ -166,11 +166,11 @@ test_expect_success 'cherry-pick fails if cwd needs to be removed' '
 '
 
 test_expect_success 'rebase does not clean cwd incidentally' '
-	test_incidental_dir_removal failure git rebase reverted
+	test_incidental_dir_removal success git rebase reverted
 '
 
 test_expect_success 'rebase fails if cwd needs to be removed' '
-	test_required_dir_removal failure git rebase fd_conflict
+	test_required_dir_removal success git rebase fd_conflict
 '
 
 test_expect_success 'revert does not clean cwd incidentally' '