Add branch querying/filtering feature

The git-branch doc states

With --contains, shows only the branches that contain the named commit (in other
words, the branches whose tip commits are descendants of the named commit).

With --merged, only branches merged into the named commit (i.e. the branches whose
tip commits are reachable from the named commit) will be listed.

With --no-merged only branches not merged into the named commit will be listed.

If the <commit> argument is missing it defaults to HEAD (i.e. the tip of the
current branch).

A potential API design might rely on the following

• Add a QueryBy() method to the BranchCollection in a similar way to the CommitCollection.QueryBy() method.
• Rename the current Filter type to CommitFilter, in order to allow the creation of a BranchFIlter

Binding the following method might also be helpful. Related libgit2 tests can be found here.

/**
 * Find a merge base between two commits
 *
 * @param out the OID of a merge base between 'one' and 'two'
 * @param repo the repository where the commits exist
 * @param one one of the commits
 * @param two the other commit
 */
GIT_EXTERN(int) git_merge_base(
     git_oid *out,
     git_repository *repo,
     git_oid *one,
     git_oid *two);

從 BranchCollection 開始，將其提議的 QueryBy() API 與 CommitCollection.QueryBy() 進行比較。在考慮提議的重新命名之前，先檢查目前的 Filter 型別，然後檢查 libgit2 的 revwalk/mergebase.c 測試以及 git_merge_base binding。完成的標準是：branch contains/merged/not-merged 查詢和相關的 merge-base API 都已定義，並附有相符的測試，同時 API 設計已經確定。