grep: fix -l/-L interaction with decoration lines

grep: fix -l/-L interaction with decoration lines

In threaded mode, git-grep emits file breaks (enabled with context, -W
and --break) into the accumulation buffers even if they are not
required.  The output collection thread then uses skip_first_line to
skip the first such line in the output, which would otherwise be at
the very top.

This is wrong when the user also specified -l/-L/-c, in which case
every line is relevant.  While arguably giving these options together
doesn't make any sense, git-grep has always quietly accepted it.  So
do not skip anything in these cases.

Signed-off-by: Albert Yale <surfingalbert@gmail.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>