fill only up to the amount we need

[spaghetti-squash]

This does two things, broadly:

1. Previously, while we checked getFuel() < targetUnits (as one would expect for a function called fillTo rather than fillBy), we were ultimately filling by targetUnits in the amount of fuel we tried to obtain.
2. this refactors the item acquisition process a little bit to avoid overshooting our fuel target, by basing the amount of fuel we obtain on the highroll. This requires more retrieve calls, but should save meat.

If we decide that the addition mall hits caused by 2 are undesirable (I would expect it to take ~3 buy-cycles for most fuels, which isn't really that much), we should close this out but still fix 1 however we see fit.