I was trying to rebuild my kernel with this patch. After updating the PKGBUILD with the patch and adding a new git apply line modeled after the existing ones, I noticed that makepkg wasn't actually applying the patch. In fact, it didn't appear to be applying any of them.
I believe this is because the kernel source code is being downloaded via a tar ball, not git, so git apply doesn't do anything (since the files to be patched are not part of a git repository). Probably the PKGBUILD should be using patch instead?