ci: Add a check that submodule changes include "Update submodule: "

To prevent repeats of https://github.com/ostreedev/ostree/pull/693

I tested this script in https://github.com/cgwalters/playground/pull/48

Closes: #770
Approved by: jlebon

.redhat-ci.Dockerfile

@@ -2,6 +2,7 @@ FROM fedora:25
 
 RUN dnf install -y \
         gcc \
+        git \
         sudo \
         which \
         attr \