further fix traceback on invalid .hgignore patterns
Please apply this little fix (matchfn return None if no patterns are
specified).
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