[merge] let tarball exports write to '-' as stdout

[Andrew Bennetts]

Martin Pool wrote:
> A semi-trivial user request.
> 
> Normally we create a root directory in the tarball (which is good
> practice) based on the name of the output file; we can't do that if we
> don't know it.  At the moment this uses '-' which is kind of
> consistent but also a bit weird.

Possibly the branch nick would be a good substitute.  The nick would need to be
filtered similiarly to "bzr send"'s default filename (IIRC), though, to avoid
characters like "/".

Or perhaps --root should just be mandatory with an output file of "-".

-Andrew.