verify: reference the correct linkrev when a filelog is missing
when a filelog is missing, the first bad revision is the first revision
where the filelog is referenced, not 0.
http://example.com/no/anchor, revs: None, checkout: None
http://example.com/an/anchor, revs: ['foo'], checkout: 'foo'
http://example.com/no/anchor/revs, revs: ['foo'], checkout: 'foo'
http://example.com/an/anchor/revs, revs: ['foo', 'bar'], checkout: 'foo'
http://example.com/an/anchor/rev-None, revs: ['foo'], checkout: 'foo'