Fix UnicodeDecodeError when both dir_limit_depth and dir_shorten_len are set

I accidentally had both set in my theme, and it causes a UniucodeDecodeError. The reason is that because of the dir_limit_depth is set, a unicode character is inserted into the segment contents. But when dir_shorten_len is set, only the first byte of the character is inserted. This causes a UnicodeDecodeError later in theme.py in the get_segments function.