forge pr list --head owner:branch does not apply --head with gitea/forgejo #111

As discovered in #109, --head is not actually implemented for gitea. So this is silently incorrect. I think we can improve upon the status quo:

• Attempt to upstream APIs https://codeberg.org/forgejo/forgejo/issues/12837
• Print warning about ignored flags
• Filter clientside

The same thing with --base afaict (though I haven't tested). There actually is a way to get specific PR from forgejo when both base and head are known, so that part could be implemented properly.