trailer: ignore first line of message

When looking for the start of the trailers in the message
we are passed, we should ignore the first line of the message.

The reason is that if we are passed a patch or commit message
then the first line should be the patch title.
If we are passed only trailers we can expect that they start
with an empty line that can be ignored too.

This way we can properly process commit messages that have
only one line with something that looks like a trailer, for
example like "area of code: change we made".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7513-interpret-trailers.sh

diff --git a/t/t7513-interpret-trailers.sh b/t/t7513-interpret-trailers.sh
index 1efb880..33cf4d1 100755
--- a/t/t7513-interpret-trailers.sh
+++ b/t/t7513-interpret-trailers.sh
@@ -93,12 +93,25 @@
 		Acked-by: Johan
 		Reviewed-by: Peff
 	EOF
-	echo "Acked-by: Johan" |
+	{ echo; echo "Acked-by: Johan"; } |
 	git -c "trailer.Acked-by.ifexists=addifdifferent" interpret-trailers \
 		--trailer "Reviewed-by: Peff" --trailer "Acked-by: Johan" >actual &&
 	test_cmp expected actual
 '
 
+test_expect_success 'with message that contains only a title' '
+	cat >expected <<-\EOF &&
+		area: change
+
+		Reviewed-by: Peff
+		Acked-by: Johan
+	EOF
+	echo "area: change" |
+	git interpret-trailers --trailer "Reviewed-by: Peff" \
+		--trailer "Acked-by: Johan" >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'with config setup' '
 	git config trailer.ack.key "Acked-by: " &&
 	cat >expected <<-\EOF &&

trailer.c

diff --git a/trailer.c b/trailer.c
index a905f5c..f7d2713 100644
--- a/trailer.c
+++ b/trailer.c
@@ -748,8 +748,9 @@
 	/*
 	 * Get the start of the trailers by looking starting from the end
 	 * for a line with only spaces before lines with one separator.
+	 * The start cannot be the first line.
 	 */
-	for (start = count - 1; start >= 0; start--) {
+	for (start = count - 1; start >= 1; start--) {
 		if (lines[start]->buf[0] == comment_line_char)
 			continue;
 		if (contains_only_spaces(lines[start]->buf)) {