GitMergeOpts: include default driver

Author: Edward Thomson

LibGit2Sharp/Core/GitMergeOpts.cs

@@ -35,6 +35,12 @@ internal struct GitMergeOpts
         /// </summary>
         public uint RecursionLimit;
 
+        /// <summary>
+        /// Default merge driver to be used when both sides of a merge have
+        /// changed.  The default is the `text` driver.
+        /// </summary>
+        public string DefaultDriver;
+
         /// <summary>
         /// Flags for automerging content.
         /// </summary>