string-list: align string_list_split() with its _in_place() counterpart

The string_list_split_in_place() function was updated by 52acddf3
(string-list: multi-delimiter `string_list_split_in_place()`,
2023-04-24) to take more than one delimiter characters, hoping that
we can later use it to replace our uses of strtok().  We however did
not make a matching change to the string_list_split() function,
which is very similar.

Before giving both functions more features in future commits, allow
string_list_split() to also take more than one delimiter characters
to make them closer to each other.

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

notes.c

@@ -892,7 +892,7 @@ static int string_list_add_note_lines(struct string_list *list,
 	 * later, along with any empty strings that came from empty
 	 * lines within the file.
 	 */
-	string_list_split(list, data, '\n', -1);
+	string_list_split(list, data, "\n", -1);
 	free(data);
 	return 0;
 }