Git: applying changes to two branches

I have two branches in my repository (for the purposes of this question): master and performance_testing. I have received changes for the master branch. I need to put them into performance_testing as well. I need to keep the two branches existent and separate, so merging would not be appropriate. I guess I could introduce the changes in one branch and commit, then do the same in the other branch. But this seems to be error-prone, and I would think that git would have some way to do this more directly. How do I do this?