Change to calculation of compile time percent increase/decrease

[Mark-Simulacrum]

Currently, ((previous - current) / current) * 100.0 is the calculation used to derive the percent change between the previous median time and the current median time for the summary page. This page on Wikipedia implies that the formula should instead be ((current - previous) / previous) * 100.0.

Wanted to open this for discussion since this change the results in my current branch from 11.4% increase in compile times to a 9.4% decrease.

[nrc]

I think I agree and we should do the latter, but it is not completely clear to me what current and previous are, so to be clear: ((week_n - week_n-1 / week_n-1) * 100.0