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

swh/lister/cgit/tests/test_lister.py

-# Copyright (C) 2019-2022 The Software Heritage developers
+# Copyright (C) 2019-2023 The Software Heritage developers
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
@@ -155,6 +155,25 @@ requests_mock_datadir_missing_url = requests_mock_datadir_factory(
 )
 
 
+def test_lister_cgit_instantiate(swh_scheduler):
+    """Build a lister with either an url or an instance is fine"""
+    url = "https://source.codeaurora.org"
+    lister = CGitLister(swh_scheduler, url=url)
+    assert lister is not None
+    assert lister.url == url
+
+    assert CGitLister(swh_scheduler, instance="source.codeaurora.org") is not None
+    assert lister is not None
+    assert lister.url == url
+
+
+def test_lister_cgit_fail_to_instantiate(swh_scheduler):
+    """Build a lister without its url nor its instance should raise"""
+    # ... It will raise without any of those
+    with pytest.raises(ValueError, match="'url' or 'instance'"):
+        CGitLister(swh_scheduler)
+
+
 def test_lister_cgit_get_origin_from_repo_failing(
     requests_mock_datadir_missing_url, swh_scheduler
 ):