builtin-clone does not fallback to copy when link fails

When cloning with the new builtin-clone, if the src repo is not on the same disk as the dest repo, cloning fails. This is because hard linking does not fall back to copying like the shell version did.

The shell version also made a distinction between defaulting to hard linking and an explicit request to hard link. In the latter case it would not fall back to copying, but would die.

I'll also mention that the 'use_local_hardlinks' variable in cmd_clone is not used. It looks like there was some initial thought that this variable would be used and then an alternative solution was found where option_no_hardlinks was accessed directly. So use_local_hardlinks should probably be cleaned out.

-brandon