[BUG?] export to tar.gz produce strange name for inner tar (was: Re: about bzr release tar.gz)

[Martin Pool]

https://bugs.launchpad.net/bzr/+bug/102234

I think this is a bug, or at least a misbehavior that we can avoid.
Python's tarfile.open has a parameter to set the file name separately
from the file object, and that's probably what goes into the headers.
We should set it to just the basename I think.