sequencer: simplify adding Signed-off-by: trailer

Add the Signed-off-by: trailer in one place rather than adding it to
the message when doing a recursive merge and specifying '--signoff'
when running 'git commit'. This means that if there are conflicts when
merging with a strategy other than 'recursive' the Signed-off-by:
trailer will be added if the user commits the resolution themselves
without passing '--signoff' to 'git commit'. It also simplifies the
in-process commit that is about to be added to the sequencer.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -477,9 +477,6 @@ static int do_recursive_merge(struct commit *base, struct commit *next,
 			_(action_name(opts)));
 	rollback_lock_file(&index_lock);
 
-	if (opts->signoff)
-		append_signoff(msgbuf, 0, 0);
-
 	if (!clean)
 		append_conflicts_hint(msgbuf);
 
@@ -657,8 +654,6 @@ static int run_git_commit(const char *defmsg, struct replay_opts *opts,
 		argv_array_push(&cmd.args, "--amend");
 	if (opts->gpg_sign)
 		argv_array_pushf(&cmd.args, "-S%s", opts->gpg_sign);
-	if (opts->signoff)
-		argv_array_push(&cmd.args, "-s");
 	if (defmsg)
 		argv_array_pushl(&cmd.args, "-F", defmsg, NULL);
 	if ((flags & CLEANUP_MSG))
@@ -1347,6 +1342,9 @@ static int do_pick_commit(enum todo_command command, struct commit *commit,
 		}
 	}
 
+	if (opts->signoff)
+		append_signoff(&msgbuf, 0, 0);
+
 	if (is_rebase_i(opts) && write_author_script(msg.message) < 0)
 		res = -1;
 	else if (!opts->strategy || !strcmp(opts->strategy, "recursive") || command == TODO_REVERT) {