repart: Rework Minimize= option settings

repart: Rework Minimize= option settings

Instead of having Minimize= take a boolean let's allow for two
different ways to enable it. "best" means we want the most minimal
image possible, which currently is only possible for read-only
filesystems but can be extended in the future with bisection
to find the most minimal possible size.

We also add "guess", which is the current behavior, where we
populate once and use the sparse size to make a reasonable guess
on a size that fits all the sources without needing to O(log(n))
tries to find the most minimal size.