apply: introduce `git_apply`
Introduce `git_apply`, which will take a `git_diff` and apply it to the
working directory (akin to `git apply`), the index (akin to `git apply
--cached`), or both (akin to `git apply --index`).
apply: introduce `git_apply_tree`
Introduce `git_apply_tree`, which will apply a `git_diff` to a given
`git_tree`, allowing an in-memory patch application for a repository.