hexdump: Partial lines cannot be repetitions of earlier lines.

[delphij]

Summary

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 <[e-mail]>
PR: 118723
MFC-after: 2 weeks

Test Plan

run tests

[emaste]

usr.bin/hexdump/display.c

274

Maybe need > 0 for style? (Even if the rest of this file isn't fully style compliant)