Ensure Tags can be created in detached Head state

Fix #791

Author: nulltoken

LibGit2Sharp.Tests/TagFixture.cs

@@ -247,6 +247,7 @@ public void CreatingATagForHeadInAEmptyRepositoryThrows()
             using (var repo = new Repository(repoPath))
             {
                 Assert.Throws<LibGit2SharpException>(() => repo.ApplyTag("mytaghead", "HEAD"));
+                Assert.Throws<LibGit2SharpException>(() => repo.ApplyTag("mytaghead"));
             }
         }
 
@@ -287,6 +288,26 @@ public void CanAddATagForImplicitHead()
             }
         }
 
+        [Fact]
+        public void CanAddATagForImplicitHeadInDetachedState()
+        {
+            string path = CloneStandardTestRepo();
+            using (var repo = new Repository(path))
+            {
+                repo.Checkout(repo.Head.Tip);
+
+                Assert.True(repo.Info.IsHeadDetached);
+
+                Tag tag = repo.ApplyTag("mytag");
+                Assert.NotNull(tag);
+
+                Assert.Equal(repo.Head.Tip.Id, tag.Target.Id);
+
+                Tag retrievedTag = repo.Tags[tag.CanonicalName];
+                Assert.Equal(retrievedTag, tag);
+            }
+        }
+
         [Fact]
         // Ported from cgit (https://github.com/git/git/blob/1c08bf50cfcf924094eca56c2486a90e2bf1e6e2/t/t7004-tag.sh#L87)
         public void CreatingADuplicateTagThrows()

LibGit2Sharp/RepositoryExtensions.cs

@@ -68,7 +68,7 @@ private static void EnsureNoGitLink<T>() where T : GitObject
         /// <param name="tagName">The name of the tag to create.</param>
         public static Tag ApplyTag(this IRepository repository, string tagName)
         {
-            return ApplyTag(repository, tagName, repository.Head.CanonicalName);
+            return ApplyTag(repository, tagName, RetrieveHeadCommitId(repository, "Cannot apply Tag.").Sha);
         }
 
         /// <summary>
@@ -91,7 +91,16 @@ public static Tag ApplyTag(this IRepository repository, string tagName, string o
         /// <param name="message">The annotation message.</param>
         public static Tag ApplyTag(this IRepository repository, string tagName, Signature tagger, string message)
         {
-            return ApplyTag(repository, tagName, repository.Head.CanonicalName, tagger, message);
+            return ApplyTag(repository, tagName, RetrieveHeadCommitId(repository, "Cannot apply Tag.").Sha, tagger, message);
+        }
+
+        private static ObjectId RetrieveHeadCommitId(IRepository repository, string message)
+        {
+            Commit commit = repository.Head.Tip;
+
+            Ensure.GitObjectIsNotNull(commit, "HEAD");
+
+            return commit.Id;
         }
 
         /// <summary>