Fix differences not shown for files without BOM preamble (close #25)

GitDiffMargin/Git/GitCommands.cs

@@ -88,7 +88,7 @@ private static byte[] GetCompleteContent(ITextDocument textDocument, ITextSnapsh
             var content = textDocument.Encoding.GetBytes(currentText);
 
             var preamble = textDocument.Encoding.GetPreamble();
-            if (preamble.Length <= 0) return null;
+            if (preamble.Length == 0) return content;
 
             var completeContent = new byte[preamble.Length + content.Length];
             Buffer.BlockCopy(preamble, 0, completeContent, 0, preamble.Length);