fix handling of files of unsupported type in the walk code if a file was of unsupported type, it was considered as 'seen' while walking. this way it was possible to have file in the dirstate not yielded by the walk function.

abort: Failed to parse .../t/.hg/hgrc
File contains no section headers.
file: .../t/.hg/hgrc, line: 1
'invalid\n'