Add Tag.PeeledTarget property

Per issue #551, a PeeledTarget property has been added to Tag. When the tag
is pointing to a chained TagAnnotation, it will return the final GitObject
that the chain points to.

closes #551

Author: bording

LibGit2Sharp.Tests/TagFixture.cs

@@ -4,6 +4,7 @@
 using LibGit2Sharp.Core;
 using LibGit2Sharp.Tests.TestHelpers;
 using Xunit;
+using Xunit.Extensions;
 
 namespace LibGit2Sharp.Tests
 {
@@ -740,6 +741,35 @@ public void LookupNullTagNameThrows()
             }
         }
 
+        [Fact]
+        public void CanRetrieveThePeeledTargetOfATagPointingToATag()
+        {
+            string path = SandboxBareTestRepo();
+            using (var repo = new Repository(path))
+            {
+                Tag tag = repo.Tags["test"];
+
+                Assert.True(tag.Target is TagAnnotation);
+                Assert.True(tag.PeeledTarget is Commit);
+            }
+        }
+
+        [Theory]
+        [InlineData("e90810b")]
+        [InlineData("lw")]
+        [InlineData("point_to_blob")]
+        [InlineData("tag_without_tagger")]
+        public void PeeledTargetAndTargetAreEqualWhenTagIsNotChained(string tagName)
+        {
+            string path = SandboxBareTestRepo();
+            using (var repo = new Repository(path))
+            {
+                Tag tag = repo.Tags[tagName];
+
+                Assert.Equal<GitObject>(tag.Target, tag.PeeledTarget);
+            }
+        }
+
         private static T[] SortedTags<T>(IEnumerable<Tag> tags, Func<Tag, T> selector)
         {
             return tags.OrderBy(t => t.CanonicalName, StringComparer.Ordinal).Select(selector).ToArray();

LibGit2Sharp/Tag.cs

@@ -40,6 +40,27 @@ public virtual GitObject Target
             }
         }
 
+        /// <summary>
+        /// Gets the peeled <see cref="GitObject"/> that this tag points to.
+        /// </summary>
+        public virtual GitObject PeeledTarget
+        {
+            get
+            {
+                GitObject target = TargetObject;
+
+                var annotation = target as TagAnnotation;
+
+                while (annotation != null)
+                {
+                    target = annotation.Target;
+                    annotation = target as TagAnnotation;
+                }
+
+                return target;
+            }
+        }
+
         /// <summary>
         /// Indicates whether the tag holds any metadata.
         /// </summary>