sequencer: reencode old merge-commit message

sequencer: reencode old merge-commit message

During rebasing, old merge's message (encoded in old encoding)
will be used as message for new merge commit (created by rebase).

In case of the value of i18n.commitencoding has been changed after the
old merge time. We will receive an unusable message for this new merge.

Correct it.

This change also notice a breakage with git-rebase label system.

Signed-off-by: Doan Tran Cong Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>