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.

从 Repository 入口点开始，检查 issue 中描述的 libgit2 git_repository_hashfile 功能。定义所要求的 HashFile 重载，包括 applyFilters 选项以及相对于仓库的路径或绝对路径，同时保留不得将文件添加到对象数据库的要求。