Optionally omit build date when creating dir-indexes

The 'use current date as the build date' feature of dirindex.py is causing slow updating of FGData, because ever single .dirindex for (say) 2024.1.3-rc2 has a different build date to the corresponding files included in the 2024.1.2 data TXZ, even if the leaf contents are unchanged.

My suggestion is to add an optional omit the comment entirely: I realise I can pass a fixed string, but a comment stating 'built on none' seems worse than committing the comment?

Due to how create_base_package.sh runs, I don't think it's feasible to preserve or include the 'old' .dirindex files, but maybe it is? That would mean getting the previous index files, and only over-writing if the child hashes have actually changed.