[MERGE] Implement hard-link support for branch and checkout

[Stefan Ring]

I'm sorry if this has been discussed before but it didn't immediately
stick out when I searched the list.

Anyway, wouldn't it make sense to hard-link the repository as well? I
would very much enjoy to have this feature. For Mercurial, the
official way to clone a directory is "cp -al". I love that! I tried it
with bzr when I started playing around with it (0.90) but it seemed to
corrupt the repositories. Also, with the knitpacks format being more
or less append-only, it should be fairly safe.

Or is it supported already maybe? I might have missed this one.