Support retrieving submodule Id in TreeEntry

Author: dahlbyk

LibGit2Sharp.Tests/SubmoduleFixture.cs

@@ -64,9 +64,17 @@ public void CanRetrieveTheCommitIdsOfASubmodule(string name, string headId, stri
                 Assert.Equal((ObjectId)headId, submodule.HeadCommitId);
                 Assert.Equal((ObjectId)indexId, submodule.IndexCommitId);
                 Assert.Equal((ObjectId)workDirId, submodule.WorkDirCommitId);
+
+                AssertEntryId((ObjectId)headId, repo.Head[name], c => c.Target.Id);
+                AssertEntryId((ObjectId)indexId, repo.Index[name], i => i.Id);
             }
         }
 
+        private static void AssertEntryId<T>(ObjectId expected, T entry, Func<T, ObjectId> selector)
+        {
+            Assert.Equal(expected, ReferenceEquals(entry, null) ? null : selector(entry));
+        }
+
         [Fact]
         public void CanEnumerateRepositorySubmodules()
         {

LibGit2Sharp/TreeEntry.cs

@@ -72,7 +72,7 @@ internal ObjectId TargetId
 
         private GitObject RetrieveTreeEntryTarget()
         {
-            if (!Type.HasAny(new[]{GitObjectType.Tree, GitObjectType.Blob}))
+            if (!Type.HasAny(new[] { GitObjectType.Tree, GitObjectType.Blob, GitObjectType.Commit }))
             {
                 throw new InvalidOperationException(string.Format(CultureInfo.InvariantCulture, "TreeEntry target of type '{0}' are not supported.", Type));
             }