Enhance the file filtering capabilities.
We now allow filtering through either pipes or pairs of temporary
files. The latter appear to be mandatory for use on Windows.
abort: repository a/.hg not found!
255
requesting all changes
abort: error: Connection refused
255
abort: repository a/.hg not found!
255
abort: destination '../a' already exists
1
abort: repository a/.hg not found!
255
abort: destination 'q' already exists