cgit: Allow url to be optional

Some cgit instances have basic urls so we can just pass the instance parameter instead.

[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
```

Refs. swh/devel/swh-lister#4693