sequencer: comment commit messages properly

sequencer: comment commit messages properly

The rebase todo editor has commands like `fixup -c` which affects
the commit messages of the rebased commits.[1]  For example:

    pick hash1 <msg>
    fixup hash2 <msg>
    fixup -c hash3 <msg>

This says that hash2 and hash3 should be squashed into hash1 and
that hash3’s commit message should be used for the resulting commit.
So the user is presented with an editor where the two first commit
messages are commented out and the third is not.  However this does
not work if `core.commentChar`/`core.commentString` is in use since
the comment char is hardcoded (#) in this `sequencer.c` function.
As a result the first commit message will not be commented out.

† 1: See 9e3cebd97cb (rebase -i: add fixup [-C | -c] command,
    2021-01-29)

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Co-authored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Reported-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>