sysupdate: Repair incomplete versions in-place
A previous commit made sysupdate recognize installed versions where some
transfers are missing. This commit teaches sysupdate how to correctly
repair these incomplete versions.
Previously, if you had a incomplete installation of the OS booted, and
ran sysupdate in an attempt to repair it, sysupdate would make things
worse by creating copies of the currently-booted partitions in the
inactive slots. Then at boot you have two identical partitions, with
identical labels an UUIDs, and end up with a mess.
With this commit, sysupdate is able to recognize situations where it can
simply download the missing transfers and leave the rest of the system
undistrubed.
Partial fix for https://github.com/systemd/systemd/issues/33339