cgit: Allow url to be optional

Some cgit instances are at a domain's root path so we can build their url directly from their 'instance' parameter.

This unifies further the cli to register a lister and the cli to schedule the listed origins from a forge [2].

[1]

https://git.kernel.org
https://source.codeaurora.org
https://git.trueelena.org
https://dev.sanctum.geek.nz
https://git.trueelena.org
https://git.dpkg.org
https://anongit.mindrot.org
https://git.aurel32.net
https://gitweb.gentoo.org
https://git.joeyh.name
https://git.adrian.geek.nz

[2] 19bdeefb

Refs. #4693