This fix conditionally registers the git prompt async handler only
if `git_prompt_info` is used anywhere in the prompt variables.
This is done in the proper order, so that the async request is
processed once the handler has been registered.
This fix also passes the return value of the previous command
to each of the async handlers, in case they are needed.
BREAKING CHANGE: the `git_prompt_info` prompt function has been
reworked by default to use the new async prompt feature. If you're
experiencing issues see #12257.
Co-authored-by: Carlo Sala <carlosalag@protonmail.com>
