git tfs rcheckin --auto-stash

Enables `--auto-stash` option for `git tfs rcheckin` that, if the working directory is dirty, will automatically wrap the `rcheckin` operation in `git stash` and `git stash pop`.

One can also add an option to the git config to enable the feature without needing to pass the command line switch.

```bash
git config git-tfs.rcheckin-autostash true
```

I tried to add integration tests for `rcheckin` to go along with this, but I think the fake VS provider needs a bit more work. I'll keep plugging away at getting that class under test: I'd like to refactor it further in later PRs.

Thanks for all you do with this project, its a life saver for me at work :smile: