Add Flags attribute to RepositoryRequiredParameters

Since we're doing bitwise operations on them, let's make sure the
compiler and runtime know that we use these as flags.

LibGit2Sharp/Repository.cs

@@ -37,6 +37,7 @@ public sealed class Repository : IRepository
         private readonly SubmoduleCollection submodules;
         private readonly Lazy<PathCase> pathCase;
 
+        [Flags]
         private enum RepositoryRequiredParameter
         {
             None = 0,