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.
Hence the suggestion to use `merge_indexes`.  It is significantly more efficient (`add_bypath` will bsearch the index for every insertion) and will not trash the stat cache, which is very important to large repositories.