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.
To be precise my understanding is that if you checkout a tree that contains only file `A` and the workdir contains `B` and `C` with `C` being ignored, then `A` will be added, `B` deleted and `C` left untouched.