copies: clean up _related logic
The limit parameter was never actually used, since the only way the 4th case
could be reached was if f1r and f2r converged. The new code makes this clear,
and additionally reduces the conditional block to just 3 cases.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])