* Investigate gz files that zgz cannot reproduce. Such files are stored
  in the testsuite git branch.
* Would be nice to keep a md5sums list of the files in the manifest and
  check and error if any are different. The error message from xdelta when
  a source file doesn't have the right checksup is fairly obscure and
  doesn't aid in fixing the problem.

  There's an implementation of this in the manifest-md5sums branch of
  git://git.debian.org/git/users/kibi-guest/pristine-tar.git , but I have
  not yet decided about applying it, since it would bloat the delta files
  with a list of all the files and md5sums..
* Optimisation: Enhance zgz so it runs multiple compression alternatives
  block-by-block in parallell, aborting compressors when they differ
  from the target file.
