Convert NoteDb migrator output into chars

Fix the NoteDb migration progress by converting its binary output
into printable characters that can be easily understood by humans.

Bug: Issue 11444
Change-Id: I6c2c6f036d8e5402ce72b7cdd53a4677b0f54521

java/com/google/gerrit/server/notedb/rebuild/NoteDbMigrator.java

@@ -142,7 +142,7 @@ public class NoteDbMigrator implements AutoCloseable {
         logger.atInfo().log(outputBuffer.toString());
         outputBuffer = new StringBuilder();
       } else {
-        outputBuffer.append(b);
+        outputBuffer.append(Character.toChars(b));
       }
     }
   }