根据documentation：
$ git show 807736c691865a8f03c6f433d90db16d2ac7a005:a.txt
等效于下面的代码：

using System;
using System.IO;
using System.Linq;
using System.Text;
using LibGit2Sharp;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            var pathToFile = "a.txt";
            var commitSha = "807736c691865a8f03c6f433d90db16d2ac7a005";
            var repoPath = @"path/to/repo";

            using (var repo =
                new Repository(repoPath))
            {
                var commit = repo.Commits.Single(c => c.Sha == commitSha);
                var file =  commit[pathToFile];

                var blob = file.Target as Blob;
                using (var content = new StreamReader(blob.GetContentStream(), Encoding.UTF8))
                {
                    var fileContent = content.ReadToEnd();
                    Console.WriteLine(fileContent);
                }
            }
        }
    }
}

正如nulltoken在注解中所说，Lookup<T>()可以使用colon-pathspec语法。

using (var repo = new Repository(repoPath))
{
    // The line below is the important one.
    var blob = repo.Lookup<Blob>(commitSha + ":" + path);

    using (var content = new StreamReader(blob.GetContentStream(), Encoding.UTF8))
    {
        var fileContent = content.ReadToEnd();
        Console.WriteLine(fileContent);
    }
}

标记线是Andrzej Gis's answer的更改。它替换commit =、file =和blob =线。此外，commitSha可以是任何refspec：v3.17.0、HEAD、origin/master等等。