Python rewrite of tar_scm

This is a rewrite of the tar_scm service in Python to improve security and
restore functionality after commit acecb041929bc8210c4165574e85bd8a6f0748b2.

Signed-off-by: Jan Blunck <jblunck@infradead.org>
that's horribly inefficient.  pls `output.append(line.rstrip())` inside the loop and `'\n'.join(output)` outside it.