Allow pull requests, and report correct commit oid and ref

.github/workflows/codeql.yml

@@ -1,6 +1,6 @@
 name: "CodeQL action"
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   build:
@@ -11,6 +11,16 @@ jobs:
 
     steps:
     - uses: actions/checkout@v1
+       with:
+         # Must fetch at least the immediate parents so that if this is
+         # a pull request then we can checkout the head of the pull request.
+         fetch-depth: 2
+
+    # If this run was triggered by a pull request event then checkout
+    # the head of the pull request instead of the merge commit.
+    - run: git checkout HEAD^2
+      if: ${{ github.event_name == 'pull_request' }}
+
     - uses: ./init
       with:
         languages: javascript