ls-remote: document the '--get-url' option

ls-remote: document the '--get-url' option

While looking for a way to expand the URL of a remote
that uses a 'url.<name>.insteadOf' config option I stumbled
over the undocumented '--get-url' option of 'git ls-remote'.
This adds some minimum documentation for that option.

And while at it, also add that option to the '-h' output.

Signed-off-by: Stefan Naewe <redacted>
Signed-off-by: Junio C Hamano <redacted>