Revert should throw if HEAD branch is orphaned

Author: jamill

LibGit2Sharp.Tests/RevertFixture.cs

@@ -434,5 +434,29 @@ public void RevertWithNothingToRevert(bool commitOnSuccess)
                 }
             }
         }
+
+        [Fact]
+        public void RevertOrphanedBranchThrows()
+        {
+            // The branch name to perform the revert on
+            const string revertBranchName = "refs/heads/revert";
+
+            string path = CloneRevertTestRepo();
+            using (var repo = new Repository(path))
+            {
+                // Checkout the revert branch.
+                Branch branch = repo.Checkout(revertBranchName);
+                Assert.NotNull(branch);
+
+                Commit commitToRevert = repo.Head.Tip;
+
+                // Move the HEAD to an orphaned branch.
+                repo.Refs.UpdateTarget("HEAD", "refs/heads/orphan");
+                Assert.True(repo.Info.IsHeadUnborn);
+
+                // Revert the tip of the refs/heads/revert branch.
+                Assert.Throws<UnbornBranchException>(() => repo.Revert(commitToRevert, Constants.Signature));
+            }
+        }
     }
 }

LibGit2Sharp/Repository.cs

@@ -1102,6 +1102,11 @@ public RevertResult Revert(Commit commit, Signature reverter, RevertOptions opti
             Ensure.ArgumentNotNull(commit, "commit");
             Ensure.ArgumentNotNull(reverter, "reverter");
 
+            if (Info.IsHeadUnborn)
+            {
+                throw new UnbornBranchException("Can not revert the commit. The Head doesn't point at a commit.");
+            }
+
             options = options ?? new RevertOptions();
 
             RevertResult result = null;