checkupdates: sort tags by date in one go

checkupdates: sort tags by date in one go

This greatly speeds up checkupdates when UCM:Tags is used on git
repositories, especially on ones with lots of tags. The old method ran a
command per tag. The new method runs a single command and doesn't
require any sorting.

Test runs show that `fdroid checkupdates org.adaway` is down from ~13s
to ~10s on my laptop.