add_excludes_from_file: clarify the bom skipping logic

Even though the previous step shifts where the "entry" begins, we
still iterate over the original buf[], which may begin with the
UTF-8 BOM we are supposed to be skipping.  At the end of the first
line, the code grabs the contents of it starting at "entry", so
there is nothing wrong per-se, but the logic looks really confused.

Instead, move the buf pointer and shrink its size, to truly
pretend that UTF-8 BOM did not exist in the input.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -576,10 +576,11 @@ int add_excludes_from_file_to_list(const char *fname,
 
 	el->filebuf = buf;
 
-	if (size >= 3 && !memcmp(buf, utf8_bom, 3))
-		entry = buf + 3;
-	else
-		entry = buf;
+	if (size >= 3 && !memcmp(buf, utf8_bom, 3)) {
+		buf += 3;
+		size -= 3;
+	}
+	entry = buf;
 
 	for (i = 0; i < size; i++) {
 		if (buf[i] == '\n') {