[MERGE] support import into empty branch.

[Aaron Bentley]

Robert Collins wrote:
> If a version is not constructable baz-import will fail and leave an
> empty branch behind.

This seems like a sloppy way to operate.  If we aren't able to complete
the operation, shouldn't we remove the empty directory?

> Likewise if someone does 'init' then 'baz-import'
> it will fail.
> 
> The former is a significant problem because it makes the archive
> unimportable - I think its nicer to treat an empty branch (no commits)
> as a new dir to import into.

Well, I don't think it makes much sense to permit importing into empty
branches, but if you want it, you can have it.

Aaron