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.
Basically just 

```
*out_stash_tree = stash_tree;
*out_base_tree = base_tree;
*out_index_tree = index_tree;
*out_index_parent_tree = index_parent_tree;
*out_untracked_tree = untracked_tree;

cleanup:

git_commit_free(...);
if (error < 0) {
    git_tree_free(...);
}