Use staging area to integrate with GitLab CI.

arc diff will push to the staging area.

Then, in .gitlab-ci.yml, configure the CI to be run for tags phabricator/diff/xxx.

• Create a pipeline trigger token: https://docs.gitlab.com/ee/ci/triggers/#create-a-pipeline-trigger-token
• Add build plan: https://lily-is.land/api/v4/projects/1071/ref/phabricator%2Fdiff%2F${buildable.diff}/trigger/pipeline?token=TOKEN&variables[TARGET_PHID]=${target.phid}
• an external system must call harbormaster.sendmessage with the build target PHID, and either "pass" or "fail" to indicate the result for this step. After the result is recorded, the build plan will resume. We use a job in the pipeline for this.

When landing, checkout that commit, arc amend then arc land.