References hg bookmarks as git branches named bookmark/<name>

[dusty-phillips]

There has been much discussion of this on #13 and #6 and I am convinced that this change will prevent future confusion down the road.

We already reference branches/ for named hg branches. Anonymous heads still need to be worked out, as well as what happens when you try to push a ref that isn't under the branches/ or bookmarks/ namespaces.