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.
Interesting.  Thanks for the background.  @peff could you shed any light on why git.git chose to do this?  I confess that I don't see the immediate benefits over simply *never* putting this into the index.