💚 check for changes found by moban

Re-instates check dropped in b8b61f8.

Use --ignore-blank-lines because sphinx templates have
trailing lines that are not wanted here, and 92b799a
and others removed the custom handling of these, and
moban doesnt yet have post-processors to handle this.

git diff --ignore-blank-lines --exit-code still emits
an exit code even if there was no applicable changes,
even with git v2.22.0 on Travis' bionic image.
Workaround with a shell loop.

Makefile

@@ -2,6 +2,8 @@ all: upstreaming
 
 upstreaming:
 	moban -m mobanfile
+	git diff
+	git diff --ignore-blank-lines | while read line; do if [ "$line" ]; then exit 1; fi; done
 
 lint: flake8 . --exclude=.moban.d,docs --builtins=unicode,xrange,long