GIT: Modifying Files and Committing with Same Date / Time

It seems that it is not easy to write a file with different content, but with the same date / time.

In the following situation:

• Create a git repository
• Add file "foo.bar" with specific creation date / time fred / lastwrite (eg 2015-01-01 00:00:00)
• Commit this file
• Change the content of "foo.bar" and set the date / time back to one value
• call "git status" => do not commit anything, working directory is inactive
• call "git commit" => No changes; do nothing

How can I force commit !?

Here is the playback code with libGit2Sharp:

using System.IO;
using LibGit2Sharp;
using System;
namespace GitWorkingUpdateProblem01
{
  class Program
  {
    static void Main(string[] args)
    {
      const string repoDir = @".\git-Test";
      Repository.Init(repoDir);
      using (var repo = new Repository(repoDir))
      {
        string fileName = Path.Combine(repo.Info.WorkingDirectory, "foo.bar");
        var dt = new DateTime(2015, 01, 01, 00, 00, 00);
        using (var sw = new StreamWriter(fileName, false))
        {
          sw.WriteLine("UNIQUE-TEXT-1234");
        }
        File.SetLastWriteTime(fileName, dt); File.SetCreationTime(fileName, dt);

        repo.Stage(fileName); repo.Commit("1");

        using (var sw = new StreamWriter(fileName, false))
        {
          sw.WriteLine("UNIQUE-TEXT-4321");
        }
        File.SetLastWriteTime(fileName, dt); File.SetCreationTime(fileName, dt);

        repo.Stage(fileName); repo.Commit("2"); // ==> THROWS: No changes; nothing to commit.
      }
    }
  }
}