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

[Martin Pool]

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.

-- 
Martin <http://launchpad.net/~mbp/>
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