Fix the old log file to work with the log parser.

src/common/logging/backend.cpp

@@ -147,7 +147,7 @@ void ColorConsoleBackend::Write(const Entry& entry) {
 
 FileBackend::FileBackend(const std::string& filename) : bytes_written(0) {
     if (FileUtil::Exists(filename)) {
-        FileUtil::Rename(filename, filename + ".old");
+        FileUtil::Rename(filename, filename + ".old.txt");
     }
 
     // _SH_DENYWR allows read only access to the file for other programs.