rebase: parse options in stuck-long mode

There is no functional change. The reason for this change is to be able
to add a new option taking an optional argument.

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
rebase: add the --gpg-sign option

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
pull: add the --gpg-sign option.

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>