lib/repo: Support hardlink conversions from bare-user to bu-only

lib/repo: Support hardlink conversions from bare-user to bu-only

Thinking about the problem of flatpak converting from `bare-user` to `bare-user-only`
"in place" by creating a new repo and doing a `pull-local`, I realized
that we can optimize this process by doing hardlinks for both metadata
and regular files.  The repo formats are *almost* compatible, the
exception being symlinks.

An earlier patch caused us to do hardlinks for metadata, this patch takes things
to the next step and special cases this specific conversion. In this case we
need to parse the source object to determine whether or not it's a symlink.

Closes: #922
Approved by: alexlarsson