sequencer: comment `--reference` subject line properly

sequencer: comment `--reference` subject line properly

`git revert --reference <commit>` leaves behind a comment in the
first line:[1]

    # *** SAY WHY WE ARE REVERTING ON THE TITLE LINE ***

Meaning that the commit will just consist of the next line if the user
exits the editor directly:

    This reverts commit <--format=reference commit>

But the comment char here is hardcoded (#).  Which means that the
comment line will inadvertently be included in the commit message if
`core.commentChar`/`core.commentString` is in use.

† 1: See 43966ab3156 (revert: optionally refer to commit in the
    "reference" format, 2022-05-26)

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