feat(git): implement async completion for `git_prompt_status` (#12319)

This is important for themes using it, since it is usually a little slower than git_prompt_info.

Also two small fixes :
 - the handler for git_prompt_info was incorrectly named _omz_git_prompt_status
 - _defer_async_git_register was kept in precmd, there is no need to call it on each prompt