export add --rebase driver allowing to rebase topic branches from one…

[MarcWeber]

… release to another

I wondered which is the best way to copy topic branches which eventually depend on each other onto another release. This code should work in at least simple cases.

 tg export --from-to release-1.0 release-1.1 \
        --prefix release-1.1-topgit-patches/ --drop-prefix release-1.0-topgit-patches/