Don't load refspecs eagerly

If the user never asks for the refspecs, we should not spend the time and memory
to load them into managed memory.

Author: carlosmn

LibGit2Sharp/RefSpecCollection.cs

@@ -1,4 +1,5 @@
-using System.Collections;
+using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Globalization;
@@ -14,7 +15,9 @@ namespace LibGit2Sharp
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class RefSpecCollection : IEnumerable<RefSpec>
     {
-        readonly IList<RefSpec> refspecs;
+        readonly Remote remote;
+        readonly RemoteSafeHandle handle;
+        readonly Lazy<IList<RefSpec>> refspecs;
 
         /// <summary>
         /// Needed for mocking purposes.
@@ -26,7 +29,10 @@ internal RefSpecCollection(Remote remote, RemoteSafeHandle handle)
         {
             Ensure.ArgumentNotNull(handle, "handle");
 
-            refspecs = RetrieveRefSpecs(remote, handle);
+            this.remote = remote;
+            this.handle = handle;
+
+            refspecs = new Lazy<IList<RefSpec>>(() => RetrieveRefSpecs(remote, handle));
         }
 
         static IList<RefSpec> RetrieveRefSpecs(Remote remote, RemoteSafeHandle remoteHandle)
@@ -48,7 +54,7 @@ static IList<RefSpec> RetrieveRefSpecs(Remote remote, RemoteSafeHandle remoteHan
         /// <returns>An <see cref="IEnumerator{T}"/> object that can be used to iterate through the collection.</returns>
         public virtual IEnumerator<RefSpec> GetEnumerator()
         {
-            return refspecs.GetEnumerator();
+            return refspecs.Value.GetEnumerator();
         }
 
         /// <summary>