Fix issue # 58

[umairvatao]

Also add randomized commit message

#58

[joryleech]

The variable setting for dates here is UNIX specific.
Perhaps something like

await execFileAsync("git", ["commit", "--quiet", "-m", commitMessage], {
  env: {
    ...process.env,
    GIT_AUTHOR_DATE: date.toString(),
    GIT_COMMITTER_DATE: date.toString()
  }
});

would be more cross-platform.