TextInputStream: move code to ReadBufferedLine()

Look at the buffer first, before trying to read from the file.  This
reduces overhead because we don't refill the buffer after every line.

src/input/TextInputStream.hxx

@@ -47,6 +47,9 @@ public:
 	 * @return a pointer to the line, or nullptr on end-of-file or error
 	 */
 	char *ReadLine();
+
+private:
+	char *ReadBufferedLine();
 };
 
 #endif