Do not traverse unindexed directories in `pijul diff -su`

This patch adds a filter to check whether we should traverse directories, which allows us to traverse them when adding files, but not when doing a simple pijul diff -su, improving the time for that command and uncluttering the output.

Created by  pmeunier  on February 8, 2024
J33DKFPM7LLK5V47GXZYJ6LO24VYA77DYHKT4QNPESABETUOVYMQC
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