collector bench_cmp subcommand incorrectly computes percentages

[nmosier]

The collector's bench_cmp subcommand, invoked via ./target/release/collector bench_cmp ..., fails to multiply the normalized change in compilation stats (e.g., compilation time) by 100 to obtain a percent change. As a result, the percent changes reported by bench_cmp are much, much smaller than they really are.

The bug is on this line, which computes (b - a) / a without multiplying by 100.