Increase the heuristic fetchDepth value to 20 for the combined repo (dotnet#750)

Author: trylek

eng/pipelines/common/checkout-job.yml

@@ -14,7 +14,7 @@ jobs:
   steps:
   - checkout: self
     clean: true
-    fetchDepth: 5
+    fetchDepth: 20
 
   - script: git bundle create $(Build.StagingDirectory)/Checkout.bundle HEAD
     displayName: Create Checkout.bundle