git-completion: add --autostash for 'git rebase'
git-completion: add --autostash for 'git rebase'

This option was added in 58794775 (rebase: implement
--[no-]autostash and rebase.autostash, 2013-05-12).

Completion of "--autosquash" has been there, but this was not;
addition of this would require people completing "--autosquash" to
type a bit more than before.

Signed-off-by: Matthieu Moy <d4d2cc2a6ec7b2aa587d4887cc5ed200f9b685a1@imag.fr>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git