diff: Fix miscounting of --check output

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>

diff.c

@@ -1013,6 +1013,7 @@ static void checkdiff_consume(void *priv, char *line, unsigned long len)
 	char *err;
 
 	if (line[0] == '+') {
+		data->lineno++;
 		data->status = check_and_emit_line(line + 1, len - 1,
 		    data->ws_rule, NULL, NULL, NULL, NULL);
 		if (!data->status)
@@ -1023,13 +1024,12 @@ static void checkdiff_consume(void *priv, char *line, unsigned long len)
 		emit_line(set, reset, line, 1);
 		(void)check_and_emit_line(line + 1, len - 1, data->ws_rule,
 		    stdout, set, reset, ws);
-		data->lineno++;
 	} else if (line[0] == ' ')
 		data->lineno++;
 	else if (line[0] == '@') {
 		char *plus = strchr(line, '+');
 		if (plus)
-			data->lineno = strtol(plus, NULL, 10);
+			data->lineno = strtol(plus, NULL, 10) - 1;
 		else
 			die("invalid diff");
 	}

t/t4015-diff-whitespace.sh

@@ -326,4 +326,13 @@ test_expect_success 'check tabs and spaces as indentation (indent-with-non-tab:
 	! git diff --check
 
 '
+
+test_expect_success 'line numbers in --check output are correct' '
+
+	echo "" > x &&
+	echo "foo(); " >> x &&
+	git diff --check | grep "x:2:"
+
+'
+
 test_done