bash: support the --autosquash option for rebase

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1386,6 +1386,7 @@ _git_rebase ()
 			--preserve-merges --stat --no-stat
 			--committer-date-is-author-date --ignore-date
 			--ignore-whitespace --whitespace=
+			--autosquash
 			"
 
 		return