readline: fix performance issue when large line in

In general situation, it works well. But when large line arrived ,
(>2M will be obviously). Because the "lineEnding.test" would run again
and again on the growing _line_buffer, time ticks and CPU run out.

Do the "lineEnding.test" only on the new arrived part, this shoud be
economical practice. When I process 10M Bytes a line, this saved me at
least 5s time.(recive a Base64 string of an Image from child process)
style: `newPartContainsEnding`