Don't load an object just to check its ID

In order to check for equality we just need to compare the ID of the
object. We do not need to look up the object but can compare their
identifiers directly.

This also requires us to modify Branch's check for the current branch,
but the new code more accurately reflects the check we do want to
perform. Namely whether HEAD points to our reference name.

Author: carlosmn

LibGit2Sharp/Branch.cs

@@ -106,7 +106,7 @@ public virtual BranchTrackingDetails TrackingDetails
         /// </value>
         public virtual bool IsCurrentRepositoryHead
         {
-            get { return repo.Head == this; }
+            get { return repo.Head.reference.TargetIdentifier == this.CanonicalName; }
         }
 
         /// <summary>

LibGit2Sharp/ReferenceWrapper.cs

@@ -16,10 +16,11 @@ public abstract class ReferenceWrapper<TObject> : IEquatable<ReferenceWrapper<TO
         /// The repository.
         /// </summary>
         protected readonly Repository repo;
+        protected readonly Reference reference;
         private readonly Lazy<TObject> objectBuilder;
 
         private static readonly LambdaEqualityHelper<ReferenceWrapper<TObject>> equalityHelper =
-            new LambdaEqualityHelper<ReferenceWrapper<TObject>>(x => x.CanonicalName, x => x.TargetObject);
+            new LambdaEqualityHelper<ReferenceWrapper<TObject>>(x => x.CanonicalName, x => x.reference.TargetIdentifier);
 
         private readonly string canonicalName;
 
@@ -40,6 +41,7 @@ protected internal ReferenceWrapper(Repository repo, Reference reference, Func<R
 
             this.repo = repo;
             canonicalName = canonicalNameSelector(reference);
+            this.reference = reference;
             objectBuilder = new Lazy<TObject>(() => RetrieveTargetObject(reference));
         }