I believe they did use pants with wool blanket bottoms. Not so sure about the roll them up part, I always figured it was to repair worn trousers.
But, remember, I am certainly not much on HC/PC, so don't make bank on anything I say regarding this sort of stuff. Want to talk about big bore muzzleloaders or making a powder horn, I'm your guy. Styles of clothes, thread counts, etc., not so much.
Personally, those trousers don't really do a lot for me.
John