This commit adds support for `--signoff` which is used to add a
`Signed-off-by` trailer to all the rebased commits. The actual
handling is left to the rebase backends.
Signed-off-by: Pratik Karki <predatoramigo@gmail.com> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>