Added git_stash_apply() and git_stash_pop() APIs

This is a tentative implementation of git_stash_apply() based on the actual Git implementation found at https://github.com/git/git/blob/master/git-stash.sh#L440.

Once you have git_stash_apply(), it's trivial to implement git_stash_pop() so I also added it.
Thinking more about it, I prefer the current implementation for 2 reasons:
- It's mirroring the actual implementation of `git apply` in Git
- There's no `git_tree*` around that represents what's currently in the workdir, which could be anything