rebase -i: permit 'drop' of a merge commit

4c063c82e9 (rebase -i: improve error message when picking merge,
2024-05-30) added advice texts for cases when a merge commit is
passed as argument of sequencer command that cannot operate with
a merge commit. However, it forgot about the 'drop' command, so
that in this case the BUG() in the default branch is reached.

Handle 'drop' like 'merge', i.e., permit it without a message.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -2683,6 +2683,7 @@ static int check_merge_commit_insn(enum todo_command command)
 		return error(_("cannot squash merge commit into another commit"));
 
 	case TODO_MERGE:
+	case TODO_DROP:
 		return 0;
 
 	default:

t/t3404-rebase-interactive.sh

@@ -2249,6 +2249,7 @@ test_expect_success 'non-merge commands reject merge commits' '
 	edit $oid
 	fixup $oid
 	squash $oid
+	drop $oid # acceptable, no advice
 	EOF
 	(
 		set_replace_editor todo &&