Update appveyor.yml

Author: SimonCropp

src/appveyor.yml

@@ -29,6 +29,17 @@ build_script:
 - dotnet build src/Polyfill.slnx --configuration Release --verbosity quiet
 - pwsh: ./src/run-tests.ps1
 - dotnet build src/PolyfillTrimmed.slnf --target PublishAll --configuration Release
+- pwsh: |
+    git diff --exit-code
+    if ($LASTEXITCODE -ne 0) {
+      Write-Error "There are unstaged changes after running tests. Please commit the generated files."
+      exit 1
+    }
+    git diff --cached --exit-code
+    if ($LASTEXITCODE -ne 0) {
+      Write-Error "There are staged changes after running tests. Please commit the generated files."
+      exit 1
+    }
 
 on_failure:
   - ps: Get-ChildItem *.received.* -recurse | % { Push-AppveyorArtifact $_.FullName -FileName $_.Name }