build-update-repo: Spawn subprocesses when generating deltas
build-update-repo: Spawn subprocesses when generating deltas

This means we can be parallel just like we were with threads,
but we're not using an enourmous amount of memory.

GeorgesStavracas/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak