Now I want to ask you about "make rebuild-stubs" command. Should it work with Digic6 firmware dump?
No. The sigfinder doesn't currently support DIGIC 6 (Thumb-2) firmware dumps.
Fixing this is not trivial, it would mean re-doing most sigfinder functionality from scratch. Plus, correctly interpreting a dump that has both Thumb and ARM instructions is much more work...
So, the only current option is finding stubs manually (the same has to be done for an ML port as they don't have a sigfinder either (FIXME)).
I wonder if this is a step toward unifying the two code-bases, or the M line has just been moved to the P&S?
I think the latter. We'll likely see less low end powershots (which probably isn't much of a problem, because those model lines have become incredibly awful lately, IMHO).