merge: drop underscore prefix from _checkunknown()
The method has been called from commands.py since
3eab42088be4
(update: just merge unknown file collisions, 2012-02-09), so drop the
underscore prefix that suggests that it's private.
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', [])