Temporary revert of the null Trees diffing feature

Author: nulltoken

LibGit2Sharp.Tests/DiffTreeToTreeFixture.cs

@@ -1,5 +1,4 @@
-using System;
-using System.IO;
+using System.IO;
 using System.Linq;
 using System.Text;
 using LibGit2Sharp.Tests.TestHelpers;
@@ -356,7 +355,7 @@ public void CanCompareTwoVersionsOfAFileWithADiffOfTwoHunks()
             }
         }
 
-        [Fact]
+        [Fact(Skip = "Not working against libgit2 debug version.")]
         public void CanCompareATreeAgainstANullTree()
         {
             using (var repo = new Repository(StandardTestRepoPath))
@@ -379,7 +378,7 @@ public void CanCompareATreeAgainstANullTree()
             }
         }
 
-        [Fact]
+        [Fact(Skip = "Not working against libgit2 debug version.")]
         public void ComparingTwoNullTreesReturnsAnEmptyTreeChanges()
         {
             using (var repo = new Repository(StandardTestRepoPath))

LibGit2Sharp/Diff.cs

@@ -70,6 +70,10 @@ internal Diff(Repository repo)
         /// <returns>A <see cref = "TreeChanges"/> containing the changes between the <paramref name = "oldTree"/> and the <paramref name = "newTree"/>.</returns>
         public virtual TreeChanges Compare(Tree oldTree, Tree newTree, IEnumerable<string> paths = null)
         {
+            //TODO: The checks below will be removed when running against next libgit2 binaries
+            Ensure.ArgumentNotNull(oldTree, "oldTree");
+            Ensure.ArgumentNotNull(newTree, "newTree");
+
             using(GitDiffOptions options = BuildOptions(paths))
             using (DiffListSafeHandle diff = BuildDiffListFromTrees(
                 oldTree != null ? oldTree.Id : null,