compact: orphaned-pack message points to closed #8572 instead of suggesting check --repair

[ThomasWaldmann]

After interrupted backups (Ctrl-C / kill -9 of borg create while it uploads), the packs written by the interrupted run are not covered by the chunk index. borg compact then says:

2.25 GB in pack files is not covered by the index; redundant copies are reclaimed on pack rewrite, reclaiming the rest is tracked in https://github.com/borgbackup/borg/issues/8572.

Problems:

• to pack or not to pack ... #8572 is closed, so pointing users there as "tracked in" is misleading.
• The space can be reclaimed today: borg check --repair followed by borg compact brought my test repo from 2.1 GB back to 36 MB. The message should tell the user that instead.

The message is in src/borg/archiver/compact_cmd.py (the unindexed block). Other code comments still reference #8572 as open work (repository.py, archiver/check_cmd.py); those might need a look, too.

Reproducer:

borg repo-create -e aes256-ocb
head -c 3000000000 /dev/urandom > big
borg create a big &  sleep 4; kill -9 %1
borg create a big
borg delete -a sh:a
borg compact -v        # -> "... not covered by the index ... #8572", repo stays large
borg check --repair
borg compact           # -> space reclaimed

Seen with borg 2.0.0b25.dev232+g73e2284a6 (master).

[ThomasWaldmann]

@mr-raj12 ^