CI: Turn on diff outputs on failed doc gen

When the doc generator fails, make the CI output the diff of how it failed.

Author: ddustin

Makefile

@@ -674,7 +674,7 @@ gen:  $(CHECK_GEN_ALL)
 
 check-gen-updated:  $(CHECK_GEN_ALL)
 	@echo "Checking for generated files being changed by make"
-	git diff --exit-code HEAD
+	git diff --text --exit-code HEAD
 
 coverage/coverage.info: check pytest
 	mkdir coverage || true