Acquire last effected commit for a git object

As discussed at https://github.com/libgit2/libgit2/issues/495 (Retrieve history for given file), https://github.com/libgit2/pygit2/issues/231 (Show last commits for Tree or Blob) and https://github.com/libgit2/libgit2sharp/issues/89 (Question: How to get the last commit that affected a given file), I'd like introduce two functions may named as `git_commit_entry_last_commit_id` and `git_commit_tree_last_commit_id`.

- [x] introduce `git_commit_entry_last_commit_id`
- [ ] introduce `git_commit_tree_last_commit_id`
- [ ] document
- [ ] test cases