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.
Also where is `untracked_tree` coming from here? You need to merge that `untracked_tree` with the tree of the workdir which AFAIK is not readily available, no?