Finer "Large Binary File" Control with plugins

[YuanhengZ]

While using jj to manage illustration source assets (like .psd, .kra), I noticed a potential area for improvement. Many of these files, though treated as binary by version control, are essentially archives (like zip files) containing structured text (e.g., XML for layer data) and image data.

Currently, simple changes like reordering layers inside a .kra file modify the internal XML, but cause the entire file to register as changed in jj. This hides the actual, often small, modification.

Could jj implement a system to track the internal content of such archive-like files instead of the opaque binary file itself? For instance, could it track changes within the embedded XML rather than the whole .kra file?

Perhaps this could be achieved via external plugins or hooks, allowing users to define custom diffing and handling logic for specific file types?

[YuanhengZ]

It is like a similar concept to git submodule but we are dealing an archive.