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.
> I would expect that your workdir matches your index (aside from unstaged changes), which is where SAFE would apply.

That's the thing though: the workdir can be in any possible state, so why not just use `GIT_CHECKOUT_SAFE_CREATE`?