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.

Start at the Repository entry point and inspect the libgit2 git_repository_hashfile functionality described in the issue. Define the requested HashFile overloads, including the applyFilters option and repository-relative or absolute paths, while preserving the requirement not to add the file to the object database.