Various merge base  (#629)

- [x] Waiting the implementation from `libgit2`, https://github.com/libgit2/libgit2/issues/2156
- [x] Waiting the update of libgit2 binaries
- [x] Introduce `git_merge_base_many` and `git_merge_base_octopus`
- [x] Introduce Enum of merge base
- [x] Add test cases
- [x] drop testId, rebase and squash, and so on