Update IndexEntry.cs

Make AssumeUnchanged virtual.

Author: zpete

LibGit2Sharp/IndexEntry.cs

@@ -33,7 +33,7 @@ public class IndexEntry : IEquatable<IndexEntry>
         /// <summary>
         /// Whether the file is marked as assume-unchanged
         /// </summary>
-        public bool AssumeUnchanged { get; private set; }
+        public virtual bool AssumeUnchanged { get; private set; }
 
         /// <summary>
         /// Gets the id of the <see cref="Blob"/> pointed at by this index entry.