Update Base SHA for Dangerfile checks (#20032)

* puts refs

* update HEAD/BASE SHA

* add 'origin/' to branches

* remove unnecessary refs to head/base

* fix case of empty string

.github/workflows/danger.yml

@@ -24,7 +24,7 @@ jobs:
           bundler-cache: true
 
       - name: Run Danger
-        run: bundle exec danger --head=${{ github.sha }} --base=${{ github.event.pull_request.base.sha }} --verbose
+        run: bundle exec danger --verbose
 
       - name: Add Danger Label
         uses: actions-ecosystem/action-remove-labels@v1

Dangerfile

@@ -3,8 +3,8 @@
 require 'ostruct'
 
 module VSPDanger
-  HEAD_SHA = `git rev-parse --abbrev-ref HEAD`.chomp.freeze
-  BASE_SHA = 'origin/master'
+  HEAD_SHA = ENV.fetch('GITHUB_HEAD_REF', '').empty? ? `git rev-parse --abbrev-ref HEAD`.chomp.freeze : "origin/#{ENV.fetch('GITHUB_HEAD_REF')}"
+  BASE_SHA = ENV.fetch('GITHUB_BASE_REF', '').empty? ? 'origin/master' : "origin/#{ENV.fetch('GITHUB_BASE_REF')}"
 
   class Runner
     def self.run