"pre-commit" hook to make sure users are committing all of their changes

People can get confused about the staging area:
- They create a file and think "git commit" will already know about it and will commit it
- They create a file, then add it, then go on to make further modifications, thinking that git now knows about it

We can solve this by telling them "Git Gud has blocked your commit because your staging area is not the same as your working directory."

No file or test is named. Start by locating the commit command and any existing pre-commit hook handling, then reproduce both staging-area examples; done means a commit is blocked when the staging area differs from the working directory and the stated explanation is shown.

Written by the indexing model from the issue text.