borg2: check --verify-data --undelete-archives can be made faster

[ThomasWaldmann]

borg check with --verify-data and --undelete-archives:

In verify_data, we do a full pass over all repo objects to verify their cryptographic authenticity. We could build a list of object IDs of archive metadata objects as a side effect there.

If we have that, we could accelerate rebuild_archives_directory using that list instead of iterating and reading all objects again.

Note: --undelete-archives is used rarely.