completion: add --autostash and --no-autostash to pull

Ideally we should only autocomplete if pull has --rebase since
they only work with it but could not figure out how to do that
and the error message of doing git pull --autostash points out
that you need --rebase so i guess it's good enough

Signed-off-by: Albert Astals Cid <albert.astals.cid@kdab.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1922,6 +1922,7 @@ _git_pull ()
 	--*)
 		__gitcomp "
 			--rebase --no-rebase
+			--autostash --no-autostash
 			$__git_merge_options
 			$__git_fetch_options
 		"