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>

builtin/grep.c

@@ -1034,8 +1034,9 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 
 #ifndef NO_PTHREADS
 	if (use_threads) {
-		if (opt.pre_context || opt.post_context || opt.file_break ||
-		    opt.funcbody)
+		if (!(opt.name_only || opt.unmatch_name_only || opt.count)
+		    && (opt.pre_context || opt.post_context ||
+			opt.file_break || opt.funcbody))
 			skip_first_line = 1;
 		start_threads(&opt);
 	}

t/t7810-grep.sh

@@ -245,6 +245,28 @@ do
 	'
 done
 
+cat >expected <<EOF
+file
+EOF
+test_expect_success 'grep -l -C' '
+	git grep -l -C1 foo >actual &&
+	test_cmp expected actual
+'
+
+cat >expected <<EOF
+file:5
+EOF
+test_expect_success 'grep -l -C' '
+	git grep -c -C1 foo >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'grep -L -C' '
+	git ls-files >expected &&
+	git grep -L -C1 nonexistent_string >actual &&
+	test_cmp expected actual
+'
+
 cat >expected <<EOF
 file:foo mmap bar_mmap
 EOF