[xa-prep-tasks] Add <GitDiff/> Task (#4374)

Overlooked in commit 2cfce14b/PR #4362.

Doh!

build-tools/xa-prep-tasks/Xamarin.Android.BuildTools.PrepTasks/GitDiff.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Linq;
+using System.Text.RegularExpressions;
+
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+
+using IOFile = System.IO.File;
+
+namespace Xamarin.Android.BuildTools.PrepTasks
+{
+	public sealed class GitDiff : Git
+	{
+		protected   override    bool        LogTaskMessages     {
+			get { return false; }
+		}
+
+		protected   override    bool        PreserveOutput     {
+			get { return false; }
+		}
+
+		protected override string GenerateCommandLineCommands ()
+		{
+			return "diff " + Arguments;
+		}
+	}
+}