Incremental Arx

[mgautierfr]

Jubako containers (and so Arx) can be composed of different content packs.
In the same time, at creation, arx is checking for duplicated content to reuse already added content if dups is found.

If we reuse pack of previous Arx, we could have incremental Arx for (almost) free.

We would need:

• A API/CMD option to specify a source of already existing packs
• Make Arx first "index" existing packs to let it checks for duplicated content.
• Make Arx create archive referencing existing content packs.

[mgautierfr]

This depends of jubako/jubako#15 as it will allow arx to fully handle entries and so reuse existing ones from an existing arx file.