dir: allow a BOM at the beginning of exclude files

Some text editors like Notepad or LibreOffice write an UTF-8 BOM in
order to indicate that the file is Unicode text rather than whatever the
current locale would indicate.

If someone uses such an editor to edit a gitignore file, we are left
with those three bytes at the beginning of the file. If we do not skip
them, we will attempt to match a filename with the BOM as prefix, which
won't match the files the user is expecting.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t7061-wtstatus-ignore.sh b/t/t7061-wtstatus-ignore.sh
index 460789b..cdc0747 100755
--- a/t/t7061-wtstatus-ignore.sh
+++ b/t/t7061-wtstatus-ignore.sh
@@ -20,6 +20,15 @@
 	test_cmp expected actual
 '
 
+test_expect_success 'same with gitignore starting with BOM' '
+	printf "\357\273\277ignored\n" >.gitignore &&
+	mkdir -p untracked &&
+	: >untracked/ignored &&
+	: >untracked/uncommitted &&
+	git status --porcelain --ignored >actual &&
+	test_cmp expected actual
+'
+
 cat >expected <<\EOF
 ?? .gitignore
 ?? actual