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>

dir.c

@@ -538,6 +538,7 @@ int add_excludes_from_file_to_list(const char *fname,
 	struct stat st;
 	int fd, i, lineno = 1;
 	size_t size = 0;
+	static const unsigned char *utf8_bom = (unsigned char *) "\xef\xbb\xbf";
 	char *buf, *entry;
 
 	fd = open(fname, O_RDONLY);
@@ -574,7 +575,12 @@ int add_excludes_from_file_to_list(const char *fname,
 	}
 
 	el->filebuf = buf;
-	entry = buf;
+
+	if (size >= 3 && !memcmp(buf, utf8_bom, 3))
+		entry = buf + 3;
+	else
+		entry = buf;
+
 	for (i = 0; i < size; i++) {
 		if (buf[i] == '\n') {
 			if (entry != buf + i && entry[0] != '#') {

t/t7061-wtstatus-ignore.sh

@@ -20,6 +20,15 @@ test_expect_success 'status untracked directory with --ignored' '
 	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