Debian contemplates patch management

> That Debian patches are all mashed up together is not true in general.

So, what would be needed nowadays to get *reliably* all the patches that 
are being applied to Debian base packages, say for the ARM architecture?

At least few years ago that was about impossible task, only way to get 
automatically correct patches (applied), was building the Debian packages 
(which requires doing this in an environment supporting that) and because 
Debian packages do not support cross-building like some other 
distributions...