Feature Request: Per commit modified_files, JSONDiffForFile, fileMatch

[glensc]

Describe the feature

Converting #1335 to a feature request

Expected behavior

There are use-cases where you want to analyse files changed by a commit:

1. you changed package.json but not yarn.lock
2. you pinned a package and didn't leave commit message body why you pinned.
3. modified source, but didn't add/update tests

and those per commit, not whole merge request.

therefore would want to have commit.modified_files, commit.JSONDiffForFile, commit.fileMatch methods:

  const packageDiff = await commit.JSONDiffForFile("package.json")

  if (packageDiff.dependencies) {
      const newDependencies = packageDiff.dependencies.added;
      if (newDependencies.includes(blacklist)) {
        fail(`${commit.sha}: Do not add ${blacklist} to our dependencies, see CVE #23`);
      }
  }
  
  const packageJson = commit.fileMatch("package.json");
  const packageLock = commit.fileMatch("yarn.lock");

  if (packageJson.modified && !packageLock.modified) {
    warn("${commit.sha}: This commit modified `package.json`, but not `yarn.lock`");
  }
  
  const testChanges = commit.modified_files.filter(filepath => filepath.match(/(test|cypress)/));
  if (testChanges.length < 1) {
    warn(`${commit.sha}: This commit does not have any changes in automated tests. Please consider adding some tests.`)
  }

[adamsosterics]

This is something that I'd also like to see.
I'd like to add a review comment on Github for each commit that is not linked to an issue, but for that I also need to provide a path to a file that has been changed in that commit (see: https://docs.github.com/en/rest/pulls/comments?apiVersion=2022-11-28#create-a-review-comment-for-a-pull-request). For this usecase it doesn't really matter which file I'm putting the comment on.
A review comment is a bit better in this case than danger's message because the PR can't be merged until all review comments are resolved.
I'd like to write a dangerfile like this:

import { danger } from "danger";

const ISSUE_REFERENCE = `https://github.com/${danger.github.thisPR.owner}/${danger.github.thisPR.repo}/issues/`;

danger.git.commits.forEach((commit) => {
  if (!commit.message.includes(ISSUE_REFERENCE)) {
    danger.github.api.pulls.createReviewComment({
      owner: danger.github.thisPR.owner,
      repo: danger.github.thisPR.repo,
      pull_number: danger.github.thisPR.number,
      commit_id: commit.sha,
      body: "question: Do these changes belong to an issue?\n\nIf so, please add a reference to the issue in the commit messages.",
      path: commit.modified_files[0], // <--- note this
      side: "RIGHT",
      subject_type: "file",
    });
  }
});