match: don't remove '.' from _includeroots
This makes _includeroots more like _fileroots and gives visitdir() a
nice symmetry in the two.
I'm hoping to later combine the two (_fileroots and _includeroots),
and having them treated similarly should make that step easier to
follow.
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', [])