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

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>