Pathname: improve compute_disk_usage

* Count .DS_Store disk usage but not file count.
* Count symlink's own disk usage instead of ignoring it.
* Count hardlinks disk usage only once.
* Add testcase.

Closes Homebrew/homebrew#50563.

Closes Homebrew/homebrew#50566.

Signed-off-by: Xu Cheng <xucheng@me.com>