Hg-Git: Git back end for mercurial

I remember years ago learning that in some way Mercurial repo functionality was a "superset" of Git, the main issue being Mercurial's "named branches" — you could not round-trip those through Git. But you can round-trip all Git stuff through Mercurial.

I wonder how this project deals with that? Or maybe something has advanced on one side or the other so now they can round-trip each other.