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.
Yes the "degenerated" tree that was stashed which only contains the untracked files. There's no "merged tree = untracked tree + workdir tree" around at this point.