config: inbox name checking matches git.git more closely
Actually, it turns out git.git/remote.c::valid_remote_nick
rules alone are insufficient. More checking is performed as
part of the refname in the git.git/refs.c::check_refname_component
I also considered rejecting URL-unfriendly inbox names entirely,
but realized some users may intentionally configure names not
handled by our WWW endpoint for archives they don't want
accessible over HTTP.