Show diff in check-extern for better debuging

Author: abravalheri

tox.ini

@@ -70,14 +70,14 @@ commands =
 
 [testenv:{vendor,check-extern}]
 skip_install = True
-allowlist_externals = sh
+allowlist_externals = git, sh
 deps =
 	path
 	cogapp
 commands =
 	vendor: python -m tools.vendored
-	vendor: sh -c "git grep -l -F '\[\[\[cog' | xargs cog -I {toxinidir} -r"  # update `*.extern`
-	check-extern: sh -c "git grep -l -F '\[\[\[cog' | xargs cog -I {toxinidir} --check"
+	sh -c "git grep -l -F '\[\[\[cog' | xargs -t cog -I {toxinidir} -r"  # update `*.extern`
+	check-extern: git diff --exit-code
 
 [testenv:generate-validation-code]
 skip_install = True