completion: add completion for --recurse-submodules=only

Command completion for 'git-push --recurse-submodules' already knows to
complete some modes. However, the recently added mode 'only' is missing.

Adding 'only' to the recognized modes completes the list of non-trivial
modes.

Signed-off-by: Cornelius Weig <cornelius.weig@tngtech.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1674,7 +1674,7 @@ _git_pull ()
 	__git_complete_remote_or_refspec
 }
 
-__git_push_recurse_submodules="check on-demand"
+__git_push_recurse_submodules="check on-demand only"
 
 __git_complete_force_with_lease ()
 {