Show total disk space freed on cleanup.

[emiliadecaudin]

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Shows total disk space freed on cleanup. Currently, Cleanup.disk_cleanup_size is only incremented during dry runs, and total disk space freed is only shown if Cleanup.disk_cleanup_size is not zero. This PR moves the @disk_cleanup_size update out of the dry_run check, and removes the bizarre provision to negate the update, since disk_usage is invariable equal to path.disk_usage, and thus @disk_cleanup_size += disk_usage - path.disk_usage never does anything.

I could not find any explanation / could not imagine a reason why this was done in the first place; please enlighten me if there actually was a reason for this. In any case, the redundant/useless code should be removed.

I was motivated to make this change after running brew cleanup, seeing a bunch of files removed, and wondering how much space that actually freed.

[RandomDSdevel]

     This may actually have been broken for a while, I think? I don't know which commit made that the case, though, unfortunately. I meant to get around to looking into that and fixing it at some point but never did, so thanks for this, @emiliadecaudin!

[Bo98]

I think the intention was the second path.disk_usage returned the new size, but it doesn't actually work that way.

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @emiliadecaudin!