copy: optionally, reproduce hardlinks from source in destination

copy: optionally, reproduce hardlinks from source in destination

This is useful for duplicating trees that contain hardlinks: we keep
track of potential hardlinks and try to reproduce them within the
destination tree. (We do not hardlink between source and destination!).

This is useful for trees like ostree images which heavily use hardlinks
and which are otherwise exploded into separate copies of all files when
we duplicate the trees.