text: Title: save git status when run commands
  Body: Sometimes after changing many versions of the code, I'm confused about how I got this result. It would be nice if allennlp could log the current git status to `serialization_dir` when running `train` command.
  
  Here is an example of a transformers record(`git_log.json`)：
  ```
  {
      "repo_id": "<git.repo.base.Repo '/data/wts/transformers/.git'>",
      "repo_sha": "b01ddc9577b87f057e163d49563ee3f74f4810cf",
      "repo_branch": "master",
      "hostname": "XXX-GPUSERVER-144"
  }
  ```