hexdump: Partial lines cannot be repetitions of earlier lines.

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
Reviewed-by: emaste
MFC-after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D40471