084ab89 coreos-download: use `parent-pkgdiff` directly from metadata

Authored and Committed by abai 3 years ago
    coreos-download: use `parent-pkgdiff` directly from metadata
    
    Previously we need to calculate the pkg diff between releases manually
    from the existing build metadata. However, this will not be the case after
    the build metadata includes parent-pkgdiff field which already calculates
    the pkgdiffs between releases using coreos-assembler. In the future, we
    should be able to completely remove the manual calculation logic from frontend
    after a few releases and revert back to the same logic as fedora-coreos-browser.
    
    Reference: https://github.com/coreos/coreos-assembler/pull/1507
    Signed-off-by: Allen Bai <abai@redhat.com>