In order to support things which might need to use the stdout FD
after checkout has completed writing a tarball to it, preserve it
by means of `os.dup()` and `os.dup2()`

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

In order to test things which write tarballs to stdout correctly,
we need to capture the binary output cleanly.  This ensures we're not
potentially mismatching encodings in and out.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

With the new support for saving the stdout FDs cleanly, and binary mode
capture in runcli, enable the tarball-to-stdout test.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

Don't close stdout when running 'checkout' to stdout

Closes #486

See merge request !905