cgit: rewrite the CGit lister

Simplify the code:
- do only inherit from ListerBase
- implement HTTP queries directly using requests
- get rid of convoluted code

Make the origin_url gathered from the git repo's "project" page instead of
building it from the 'url_prefix' hack. Now, the lister WILL make substancially
more requests, since it will make one request per listed git repo, but
the provided origin_url should be pretty reliable now.

When several url are provided as clonable URLs, choose the http/https one first,
otherwise, choose the first one of the list.

Add proper tests for the cgit lister.

Also, get rid of the 'time_updated' column in the model.

swh/lister/cli.py

@@ -19,10 +19,6 @@ SUPPORTED_LISTERS = ['github', 'gitlab', 'bitbucket', 'debian', 'pypi',
 DEFAULT_BASEURLS = {
     'gitlab': 'https://gitlab.com/api/v4/',
     'phabricator': 'https://forge.softwareheritage.org',
-    'cgit': (
-        'http://git.savannah.gnu.org/cgit/',
-        'http://git.savannah.gnu.org/git/'
-    ),
 }
 
 
@@ -132,13 +128,8 @@ def get_lister(lister_name, db_url, drop_tables=False, **conf):
     elif lister_name == 'cgit':
         from .cgit.models import ModelBase
         from .cgit.lister import CGitLister
-        if isinstance(api_baseurl, str):
-            _lister = CGitLister(url=api_baseurl,
-                                 override_config=override_conf)
-        else:  # tuple
-            _lister = CGitLister(url=api_baseurl[0],
-                                 url_prefix=api_baseurl[1],
-                                 override_config=override_conf)
+        _lister = CGitLister(url=api_baseurl,
+                             override_config=override_conf)
 
     elif lister_name == 'packagist':
         from .packagist.models import ModelBase  # noqa