git merge already allows us to sign commits, and git rebase has recently
learned how to do so as well. Teach git pull to parse the -S/--gpg-sign
option and pass this along to merge or rebase, as appropriate.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>