git: allow cloning from the URL root, too

This means we can still show non-git users a somewhat browseable
URL with a link to the README.html file while allowing git users
to type less when cloning.

All of the following are supported:

git clone https://public-inbox.org/ public-inbox
git clone https://public-inbox.org/public-inbox
git clone https://public-inbox.org/public-inbox.git
torsocks git clone http://ou63pmih66umazou.onion/public-inbox