git-snapshot should restore index

```
$ git status --short
MM CHANGELOG.md
M  CONTRIBUTING.md
 M README.md
$ git snapshot
$ git status --short
 M CHANGELOG.md
 M CONTRIBUTING.md
 M README.md
```

Using `--index` when applying the stash will preserve the index.

```
$ git reset --hard
HEAD is now at 961a730 staged with to tuck
$ git stash apply --index --quiet
$ git status --short
MM CHANGELOG.md
M  CONTRIBUTING.md
 M README.md
```
