Increase fetchDepth to prevent checkout failure

like https://dev.azure.com/rubylang/ruby/_build/results?buildId=1637

azure-pipelines.yml

@@ -22,7 +22,7 @@ jobs:
       sudo apt-get build-dep ruby2.3
     displayName: "Install dependencies"
   - checkout: self
-    fetchDepth: 10
+    fetchDepth: 20
   - script: |
       autoconf
       ./configure