php.yml: Set actions/checkout fetch-depth to 2

Author: reedy

.github/workflows/php.yml

@@ -19,6 +19,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+      with:
+        fetch-depth: 2
 
     - name: Setup PHP
       uses: shivammathur/setup-php@v2