Add command for checking if any updates available in the submodules

How do I know if my submodule is updated?


>If you want to check for new work in a submodule, you can go into the directory and run git fetch and git merge the upstream branch to update the local code. Now if you go back into the main project and run `git diff --submodule` you can see that the submodule was updated and get a list of commits that were added to it.
