gitweb: Add optional base_git_url parameter to lister

Similar to cgit, it exist cases where git clone URLs for projects hosted on a gitweb instance cannot be found when scraping project pages or cannot be easily derived from the gitweb instance root URL.

So add an optional base_git_url parameter enabling to compute correct clone URLs by appending project names to it.

Some examples where clone URLs cannot currently be successfully computed by the lister:

• projects hosted on http://hgm.nubati.net have clone URLs in the form http://hgm.nubati.net/git/{project_name} but the lister currently outputs URLs in the form git://hgm.nubati.net/{project_name}

• projects hosted on https://gitweb.factorcode.org have clone URLs in the form https://factorcode.org/git/{project_name} but the lister currently outputs URLs in the form git://gitweb.factorcode.org/{project_name}

• projects hosted on https://gitweb.stoutner.com have clone URLs in the form https://git.stoutner.com/{project_name} but the lister currently outputs URLs in the form git://gitweb.stoutner.com/{project_name}