Implement hashing of files without adding it to the object db

Libgit2sharp does not seem to provide the equivalent functionality to git hash-object, even though libgit2 implements it. At first glance, it seems relatively simple to add, and I was planning on working on it and submitting a pull request. I was thinking that there would be a method on Repository that relies on
git_repository_hashfile and the usage would look like

using (var repo = new Repository("/repo"))
{
    string sha;
    sha = repo.HashFile("relative/path.txt");
    sha = repo.HashFile("relative/path.txt", applyFilters: false);
    sha = repo.HashFile("/repo/file.txt");
}

Any feedback is welcome.

從 Repository 入口點開始，檢查 issue 中描述的 libgit2 git_repository_hashfile 功能。定義所要求的 HashFile 多載，包括 applyFilters 選項以及相對於儲存庫的路徑或絕對路徑，同時保留不得將檔案新增至物件資料庫的要求。