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

[John Arbash Meinel]

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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.
|
|

I would like a test for what happens if you don't supply '--root'. Otherwise:

BB:tweak

John
=:->

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