I can't find anything in the manpage about git-am checking the patch file's parent(s), but a patch file is accepted by git-apply as well, and then it's certainly not expected to consider a parent. That said, I've had git-am fail while git-merge or git-rebase would do the right thing, so git-am is probably, judging by the docs as well, not taking the history into account. If it does and I forgot to tell it how, I didn't know what options it is.