After you merge everything you could just diff the originally committed files against the deployed and tested versions. If there are any differences, then those

After you merge everything you could just diff the originally committed files against the deployed and tested versions. If there are any differences, then those need testing! Presumably if you're merging a bunch of other changes in anyway, you're going to need to do a bunch of testing in any case, that's just an inevitable result of splitting the deployment into two phases.