Plugins should be provided with git diff

This is a feature request to provide the output of changed files, such as `git diff --cached --name-only --diff-filter=ACM`. The purpose of this feature is to provide a way to verify if changes described within a conventional commit's `scope` are actually all within that scope.

## Expected Behavior

Naive example:
```js
plugin: {
rules: {
'bounded-scope': (parsed) => {
console.log(parsed.diff);
const scope = parsed.scope;

if (scope === 'test1') {
changedStartsWithScope = parsed.diff.every((path) => path.startsWith('apps/test1'));
return [changedStartsWithScope, 'all files with scope test1 should start with apps/test1'];
}

return [true];
}
}
}

echo "feat(test1): random subject" | npm run commitlint
[ '.gitignore', 'apps/test1/testing.js' ]
⧗ input: feat(test1): random subject
✖ all files with scope test1 should start with apps/test1 [scope-bounded]

✖ found 1 problems, 0 warnings
```

## Current Behavior

No way of determining which files have been changed by the commit.

## Affected packages

- [ ] cli
- [x] core
- [ ] prompt
- [ ] config-angular

## Context

I'm trying to create a plugin which queries Nx to determine if the changes in the commit match the scope provided, otherwise requiring you to not specify a scope.

Start in the core package and trace how plugin rules receive parsed commit data. The request proposes exposing paths from `git diff --cached --name-only --diff-filter=ACM` as `parsed.diff`; done means a plugin can inspect those paths and enforce the scope example, with core tests covering the exposed data.

Written by the indexing model from the issue text.