When checking for repetitions of earlier lines, we compare the
first nread bytes of the line against the saved line. However,
when we read a partial line, it should never be treated as a
repetition of an earlier line, even if the first bytes match.
This change fixes a bug where a partial line could be
incorrectly identified as a repetition of an earlier line.
Reported-by: Mark Adler <madler@alumni.caltech.edu>
PR: 118723
MFC-after: 2 weeks