merge benchmark results and upload to GitHub Pages

[crazy-max]

relates to #13

Merge benchmarks, generate graphs and upload to GitHub Pages.

TODO:

• Needs to review testconfig for metrics value based on its unit to make graphs relevant
• ~Move ref_timestamp out of benchmark metrics (this is just used for sorting refs in graphs)~
• testutil.MergeBenchmarks should return an object that can be ingested directly by format output of merge command.

See https://moby.github.io/buildkit-bench/

[crazy-max]

Damn didn't want to merge that :see_no_evil:

[crazy-max]

Reverted on master branch and opened https://github.com/moby/buildkit-bench/pull/29