Improve repack.sh

[BrainStone]

Quite a few changes were done.

I split them into fairly small commits so the changes are easier to follow. Each commit has a message explaining the details.

Feel free to squash the commits, if you're not a fan of many small commits, but I ask you to keep the explanations.

All changes are up for discussion and I'm aware some are certainly more opionion based than others.
My main motivation was to use the better compression and to properly use find. The other changes were added just on top because I like to tidy up bash scripts I touch.