Adjust GitStatusOptions to match structure of native libgit2

This fixes a marshaling bug that causes a segmentation fault

Author: Molytho

Diff for: LibGit2Sharp/Core/GitStatusOptions.cs

@@ -13,6 +13,8 @@ internal class GitStatusOptions : IDisposable
 
         public GitStrArrayManaged PathSpec;
 
+        public IntPtr Baseline = IntPtr.Zero;
+
         public void Dispose()
         {
             PathSpec.Dispose();