Allow cherry-pick (and revert) to add signoff line

Allow cherry-pick (and revert) to add signoff line

I often find myself pulling patches off of other peoples trees using
cherry-pick, and following it with an immediate 'git commit --amend -s'
command.  Eliminate the need for a double commit by allowing signoff on a
cherry-pick or revert.

Signed-off-by: Dan McGee <dpmcgee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>