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

Jenkins job DLS/gitlab-builds #64 succeeded .
See Console Output and Coverage Report for more details.

Datapoints:

2023-05-22 17:08:20 swh-scheduler@db1:5432 λ select status, arguments#>>'{kwargs,url}' as url from task where type='list-cgit' and status !='disabled';
+-----------+--------------------------------------------+
|  status   |                    url                     |
+-----------+--------------------------------------------+
| completed | https://source.codeaurora.org/             |
| completed | https://git.replicant.us/replicant         |
| completed | https://git.replicant.us/infrastructure    |
| completed | https://git.replicant.us/infrastructure    |
| completed | https://git.replicant.us/replicant-next    |
| completed | https://git.replicant.us/mirrors/AOSP      |
| completed | https://git.replicant.us/mirrors/LineageOS |
| completed | https://git.trueelena.org/                 |
| completed | https://git.trueelena.org/                 |
| completed | https://dev.sanctum.geek.nz/               |
| completed | https://git.dpkg.org/                      |
| completed | https://anongit.mindrot.org/               |
| completed | https://git.aurel32.net                    |
| completed | https://git.aurel32.net                    |
| completed | https://gitweb.gentoo.org/                 |
| completed | https://git.joeyh.name/                    |
| completed | https://git.adrian.geek.nz/                |
+-----------+--------------------------------------------+
(17 rows)

Time: 48.021 ms

It wasn't clear to me what "Some cgit instances have basic urls" means without reading the code. Could you replace these words with "Some cgit instances are at a domain's root path"?

Also, you should amend that comment, because it's not just for backward compatibility anymore

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