Add CodeQL security scanning (#346)

.github/workflows/codeql.yml

@@ -1,24 +1,30 @@
-name: "Code Scanning - Action"
+name: "Code scanning - action"
 
 on:
   push:
+  pull_request:
   schedule:
-    - cron: '0 0 * * 0'
+    - cron: '0 19 * * 0'
 
 jobs:
   CodeQL-Build:
 
-    strategy:
+    # CodeQL runs on ubuntu-latest and windows-latest
-      fail-fast: false
-
-
-    # CodeQL runs on ubuntu-latest, windows-latest, and macos-latest
     runs-on: ubuntu-latest
 
     steps:
     - name: Checkout repository
       uses: actions/checkout@v2
+      with:
+        # We must fetch at least the immediate parents so that if this is
+        # a pull request then we can checkout the head.
+        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' }}
+      
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL
       uses: github/codeql-action/init@v1
@@ -27,9 +33,20 @@ jobs:
       #   languages: go, javascript, csharp, python, cpp, java
 
     # Autobuild attempts to build any compiled languages  (C/C++, C#, or Java).
-    # If this step fails, then you should remove it and run the build manually (see below).
+    # If this step fails, then you should remove it and run the build manually (see below)
     - name: Autobuild
       uses: github/codeql-action/autobuild@v1
 
+    # ℹ️ Command-line programs to run using the OS shell.
+    # 📚 https://git.io/JvXDl
+
+    # ✏️ If the Autobuild fails above, remove it and uncomment the following three lines
+    #    and modify them (or add more) to build your code if your project
+    #    uses a compiled language
+
+    #- run: |
+    #   make bootstrap
+    #   make release
+
     - name: Perform CodeQL Analysis
       uses: github/codeql-action/analyze@v1