backup: fix needed space calculation
backup: fix needed space calculation

It wasn't taking excludes into account. Now its calculation mirrors the
zip file creation.

foosel/OctoPrint,Jaesin/OctoPrint,Jaesin/OctoPrint,Jaesin/OctoPrint,foosel/OctoPrint,foosel/OctoPrint,Jaesin/OctoPrint,foosel/OctoPrint