This lets you set a prefix for the resulting archive patsh.
Especially useful in combination with --subpath, for instance
--subpath=subdir --prefix=subdir to extract just subdir.
Closes: #265
Approved by: cgwalters
At the moment I'm looking at using rpm-ostree to manage RPM inputs
which can then be converted into Docker images. It's most convenient
if we can stream directly out of libostree rather than doing a
checkout + tar combination.
There are also backup/debugging etc. reasons to implement `export` as
well.