Sure there are multiple commits on a branch, but when we merge back to the trunk the whole user story has to go in one shot. We can't put a partial user story on the trunk because then it wouldn't be in a fit state to release.
Well, we can't commit half a visible feature, for sure, but my sense is that the parts that are too complicated to review in a single setting often either involve a preparatory refactoring or represent a code base that has excessive duplication.
There are things that just can't be split apart, I admit. I had to do a truly heinous refactoring of some code awhile back to migrate from floats to doubles that couldn't be accomplished via a global find and replace (unsafe casts and other crap). That was the largest commit I've ever landed, and the code wouldn't compile for half the time I worked in that branch. Come to think of it, that refactoring was harder than it had to be because of duplication and badly written code, but it would've been massive no matter what.
