"git gud commit" should not commit user added files.

"git gud commit" should only ever commit the files we expect it to, not ones added by users.

If someone has modified the index ("git add" "git rm" etc), then we'll also be committing those extra changes when running "git gud commit"

Example:
After running these commands
```
touch foo
git add foo
git gud commit
```
This is what we see
```
$ git ls-files foo
foo
```

This is what we want to see (no output)
```
$ git ls-files foo
```

This goes along with #39 and #131. We want to limit the user to only committing in ways we want them to