save git status when run commands

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"
}