BugFix: Factor should be calculated per changeset

Factor was being calculated and as we iterate through each file in
changeset, we were changing factor to (1 - Factor). Factor once
calculated, should be same for each file in the changeset.
BugFix: Correct the formula to calculate hotspot_factor

Actual formula for calculating hotspot is 1/(1 + e ^(-12Ti + 12)).
But we were calculating it wrongly. Our hotspot value was always less
than 1/13 = ~0.07.