sequencer: comment checked-out branch properly

sequencer: comment checked-out branch properly

`git rebase --update-ref` does not insert commands for dependent/sub-
branches which are checked out.[1]  Instead it leaves a comment about
that fact.  The comment char is hardcoded (#).  In turn the comment
line gets interpreted as an invalid command when `core.commentChar`/
`core.commentString` is in use.

† 1: See 900b50c242 (rebase: add --update-refs option, 2022-07-19)

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>