Draft: Add basic support for Gitweb in the cgit lister

While looking into #4690 (closed), I poked at the cgit lister a bit until it worked on one Gitweb instance.

TODO:

• unbreak cgit tests
• add gitweb-specific tests
• check if pagination exists in gitweb, adapt the code if it does
• check that no cgit instance is broken by the changes

Ref. #1800 (moved) #4690 (closed)