lib/commit: Don't copy xattrs for metadata objects

lib/commit: Don't copy xattrs for metadata objects

Copying the xattrs on metadata objects is wrong in general, we
don't "own" them.  Notably this would fail in the situation of
doing a pull from e.g. a `bare-user` source to a destination
that was on a different mount point (so we couldn't hardlink),
and the source had e.g. a `security.selinux` attribute.

Closes: #1734
Closes: #1736
Approved by: jlebon