Suppress or fix some unused varible warnings

These (except for the one) are there to keep the pointers alive, so we
do want them there even if we never read from them.

Author: carlosmn

LibGit2Sharp.Tests/SmartSubtransportFixture.cs

@@ -84,7 +84,7 @@ public void CanUseCredentials(string scheme, string url, string user, string pas
             string remoteName = "testRemote";
 
             var scd = BuildSelfCleaningDirectory();
-            var repoPath = Repository.Init(scd.RootedDirectoryPath);
+            Repository.Init(scd.RootedDirectoryPath);
 
             SmartSubtransportRegistration<MockSmartSubtransport> registration = null;
 

LibGit2Sharp/Core/NativeMethods.cs

@@ -14,7 +14,10 @@ internal static class NativeMethods
     {
         public const uint GIT_PATH_MAX = 4096;
         private const string libgit2 = NativeDllName.Name;
+        // This is here to keep the pointer alive
+        #pragma warning disable 0414
         private static readonly LibraryLifetimeObject lifetimeObject;
+        #pragma warning restore 0414
         private static int handlesCount;
 
         /// <summary>

LibGit2Sharp/RefSpec.cs

@@ -12,7 +12,10 @@ namespace LibGit2Sharp
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class RefSpec
     {
+        // This is here to keep the pointer alive
+        #pragma warning disable 0414
         readonly Remote remote;
+        #pragma warning restore 0414
         readonly GitRefSpecHandle handle;
 
         internal RefSpec(Remote remote, GitRefSpecHandle handle)

LibGit2Sharp/RefSpecCollection.cs

@@ -15,8 +15,11 @@ namespace LibGit2Sharp
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class RefSpecCollection : IEnumerable<RefSpec>
     {
+        // These are here to keep the pointer alive
+        #pragma warning disable 0414
         readonly Remote remote;
         readonly RemoteSafeHandle handle;
+        #pragma warning restore 0414
         readonly Lazy<IList<RefSpec>> refspecs;
 
         /// <summary>