sequencer: pass explicit --no-gpg-sign to merge

The merge subcommand launched for merges with non-default strategy would
use its own default behaviour to decide how to sign commits, regardless
of what opts->gpg_sign was set to. For example the --no-gpg-sign flag
given to rebase explicitly would get ignored, if commit.gpgsign was set
to true.

Fix the issue and add a test case excercising this behaviour.

Signed-off-by: Samuel Čavoj <samuel@cavoj.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -3678,6 +3678,8 @@ static int do_merge(struct repository *r,
 		strvec_push(&cmd.args, git_path_merge_msg(r));
 		if (opts->gpg_sign)
 			strvec_pushf(&cmd.args, "-S%s", opts->gpg_sign);
+		else
+			strvec_push(&cmd.args, "--no-gpg-sign");
 
 		/* Add the tips to be merged */
 		for (j = to_merge; j; j = j->next)

t/t3435-rebase-gpg-sign.sh

@@ -89,4 +89,11 @@ test_expect_success 'rebase -r, merge strategy, commit.gpgsign=false --gpg-sign
 	git verify-commit HEAD
 '
 
+test_expect_success "rebase -r, merge strategy, commit.gpgsign=true --no-gpg-sign won't sign commit" '
+	git reset --hard merged &&
+	git config commit.gpgsign true &&
+	git rebase -fr --no-gpg-sign -s resolve --root &&
+	test_must_fail git verify-commit HEAD
+'
+
 test_done