git-debrebase: Rename new-upstream-v0 command to new-upstream

git-debrebase: Rename new-upstream-v0 command to new-upstream

This UI is pretty good now, I think, for single-piece upstreams.

For multi-piece ones, which are quite tricky anyway, it is tolerable,
and we it doesn't steal much of the available syntax space.  So we
don't need the -v0 any more.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>