Fixed commerce loss being ignored in AI_jobChangeValue.

The code was missing a minus sign, which caused commerce loss values to
be negative, which essentially meant they were ignored by AI_yieldValue.

I suppose the effect of this bug is that specialists would be much more
likely to be kept once they were assigned.
Adjusted value of food in AI_yieldValue to avoid overvalue problems.

The previous value system sometimes resulting in high value plots being
left unworked because those same high value plots were causing the
growth value of other plots to be even higher value.

Essentially the problem is caused by the casual overestimating of growth
value. So to fix it, I've reduced the base growth value to a more
natural (and lower) calculation, and made some other food value
adjustments to compensate the change.
This is version 1.44b.

Updated the changelog.