Summary:
When modifying code that lacks alternative variations, there is only a 'HEAD' benchmark - so it can be useful to keep a version of these results for latter printing in the format of a relative benchmark.

Add a new flag to simplify this use-case,
--bm_relative_to: A flag taking the path to a JSON-verbose formatted dump, which will be used when printing (in non-JSON-mode) to produce output relative to the provided prior dump.

Usage:
- generate a JSON benchmark dump, use "--bm_json_verbose"
  $ your_benchmark_binary --benchmark --bm_json_verbose  old-json
- run a benchmark printing a comparison to an old dump:
  $ your_benchmark_binary --benchmark --bm_relative_to old-json

Note: You can also use bm_json_verbose and bm_relative_to at the same time, this will print relative results and update the JSON dump

Reviewed By: luciang

Differential Revision: D26214397

fbshipit-source-id: 8134b74604b7bb5037934205e8eee5e3ffc27f15