When exporting, use hardlinks for duplicated files

When exporting, use hardlinks for duplicated files

For ostree_repo_export_tree_to_archive(), and 'ostree export', when the
exported tree contains multiple files with the same checksum, write an
archive with hard links.

Without this, importing a tree, then exporting it again breaks
hardlinks.

As an example of savings: this reduces the (compressed) size of the
Fedora Flatpak Runtime image from 1345MiB to 712MiB.

Resolves: #2925