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