Fix: [git] JSONDiffForFile when passed the path of a JSON File that was moved, crashes

[fbartho]

Don't crash when danger.git.JSONDiffForFile encounters a moved JSON file

Notably affects danger-plugin-yarn; when a package.json is moved, it slips by a test in JSONPatchForFile that filters for danger.git.modified_files, and I couldn't figure out how to fix getFileContents so it could look up the old file's contents using the new file path.

Note: I also couldn't figure out how to generate the right fixtures, or how to mock JSONPatchForFile

Fixes: #1181
Related: #481

Some useful test data

const diff = {
  before: null,
  after: {
    name: '@fbartho/something',
    description: 'this is irrelevant',
    version: '1.0.0',
    private: true,
    main: './dist/main.js',
    dependencies: { express: '^4.17.1' }
  },
}

const outerDiff = [
    { op: 'add', path: '/name', value: '@fbartho/something' },
    {
      op: 'add',
      path: '/description',
      value: 'this is irrelevant'
    },
    { op: 'add', path: '/version', value: '1.0.0' },
    { op: 'add', path: '/private', value: true },
    { op: 'add', path: '/main', value: './dist/main.js' },
    { op: 'add', path: '/dependencies', value: [Object] },
  ]