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.
I'm not certain that I'm reading this correctly, but my expectation here is that if we want to apply a tree that contains a few files, that we would merge the two trees (using `git_merge_trees`?) and then check the result.