before and after sample mode

[jeffbcross]

For purposes of calculating more accurate deltas between CLs, and accounting for different test environments, benchpress should support running tests for current revision and previous revision to calculate relative results as well as absolute results.

I'm opening this issue as a reminder that this should be supported, though it may not make sense as part of the existing benchpress stack (e.g. maybe it makes more sense as a third-party script).