No need to find merge base.

As discussed in https://github.com/libgit2/libgit2/issues/2227, finding merge-base does not help in getting performance improvement. In this PR, I have done following changes:
1) Do not call `git_merge__bases_many` in `prepare_walk`
2) Do not call `enqueue` for an uninteresting node.
3) Do not call `process_parent_commits` for an uninteresting nodes (parent of an uninteresting nodes are also uninteresting).
4) Add some more tests for revwalk.