borg-insert-update-message doesn't detect new drones and crashes on removals

[Thibault Polge]

Hi,

borg-insert-update-message:

• doesn't differentiate between a newly installed package and an updated one, and describes them both as updated.
• fails (while trying to cd to a now-non-existing directory) if a drone got removed.

Thanks!

[Jonas Bernoulli]

The command just learned how to handle those things.