That gives us a way to test e.g. pull requests by specifying
ph_scripts_refspec="pull/123/head"
Also keep "ph_" prefix to remove confusion we had between
"scripts_branch" and "ph_scripts_branch" before.
Update docs and moved "testing" sections up.
- configured sccache on buildkite windows machines
- final result is sent to phabricator from "summary.py" that waits for both builds to complete
- extracted "add_url_artifact" to a runnable script
- reorganized code and fixed some of TODOs