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

This option was added in 5879477 (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 <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1659,6 +1659,7 @@ _git_rebase ()
 			--committer-date-is-author-date --ignore-date
 			--ignore-whitespace --whitespace=
 			--autosquash --fork-point --no-fork-point
+			--autostash
 			"
 
 		return