uniq: Fix off-by-one bug in -cD case.

When printing only duplicated lines, the first line of each set is not
printed until we encounter the second. When that happens, we need to
increment the repetition count between printing the first and the
second line, so that if we are also printing counts, we don't print the
same (pre-increment) count twice.

MFC after: 1 week
PR: 275764
Reported by: Yu-Sheng Ma <s110062131@m110.nthu.edu.tw>
Submitted by: Daniel Tameling <tamelingdaniel@gmail.com> (original patch)
Sponsored by: Klara, Inc.
Reviewed by: tamelingdaniel_gmail.com, asomers, emaste
Differential Revision: https://reviews.freebsd.org/D48000