Add submodule support to Index.Stage()

Fix #220

Author: dahlbyk

LibGit2Sharp.Tests/SubmoduleFixture.cs

@@ -119,6 +119,27 @@ public void CanStageChangeInSubmoduleViaSubmoduleStage(string submodulePath)
             }
         }
 
+        [Theory]
+        [InlineData("sm_changed_head")]
+        // [InlineData("sm_changed_head/")] // AmbiguousSpecificationException - Do we want to support this?
+        public void CanStageChangeInSubmoduleViaIndexStage(string submodulePath)
+        {
+            var path = BuildTemporarySubmoduleClone();
+
+            using (var repo = new Repository(path))
+            {
+                var submodule = repo.Submodules[submodulePath];
+
+                var statusBefore = submodule.RetrieveStatus();
+                Assert.Equal(SubmoduleStatus.WorkDirModified, statusBefore & SubmoduleStatus.WorkDirModified);
+
+                repo.Index.Stage(submodulePath);
+
+                var statusAfter = submodule.RetrieveStatus();
+                Assert.Equal(SubmoduleStatus.IndexModified, statusAfter & SubmoduleStatus.IndexModified);
+            }
+        }
+
         public string BuildTemporarySubmoduleClone()
         {
             var submodule = Path.Combine(ResourcesDirectory.FullName, "submodule_wd");

LibGit2Sharp/Index.cs

@@ -172,7 +172,15 @@ public virtual void Stage(IEnumerable<string> paths)
                 }
                 else
                 {
-                    AddToIndex(relativePath);
+                    var submodule = repo.Submodules[relativePath];
+                    if (submodule != null)
+                    {
+                        submodule.Stage();
+                    }
+                    else
+                    {
+                        AddToIndex(relativePath);
+                    }
                 }
             }