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.
> You just checked it out

Where did you see that, it's not supposed to check it out. It's reinstating the index, not touching the workdir.