SkQP: new tool: tools/skqp/find_commit_with_best_gold_results

I already used this to choose where to branch SkQP for Q-release.

No-Try: true
Change-Id: Id809adefe038d4a96a47c19f7844812ea8854d1e
Reviewed-on: https://skia-review.googlesource.com/c/184920
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Hal Canary <halcanary@google.com>