c1795bb (Unify whitespace checking) incorrectly made the
checking function return without incrementing the line numbers
when there is no whitespace problem is found on a '+' line.
This resurrects the earlier behaviour.
Noticed and reported by Jay Soffian. The test script was stolen
from Jay's independent fix.
Signed-off-by: Junio C Hamano <gitster@pobox.com>