Bugfix: Calculate actual delta in usage

[tueksta]

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I read the guidelines three times, I do not understand it. Most of it seems to apply to formula changes only? Maybe needs better headline/section structure.

Changes:
This fixes a bug, that when you do a cleanup and the cleanup fails for a path (e.g. no permissions to remove path), the size of that path would still be added to the cleanup estimate. By calculating the delta of disk usage before and after the removal, we get a clear picture of how much disk space was saved.

[reitermarkus]

the cleanup fails for a path (e.g. no permissions to remove path)

Wouldn't it abort completely in that case?

[tueksta]

No, that's how I discovered this bug. Kept saying 15MB were freed, and that it couldn't delete python 3.7.2 and that python has 15MB in the path. So I wondered if it means the 15MB were freed by something else, but all successive rerans it still kept saying 15MB were cleaned and that it couldn't delete python 3.7.2.

Comments addressed.

[MikeMcQuaid]

Closed and reopened to re-trigger CI.

[MikeMcQuaid]

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