firstcommit | Dig up the first commit of any GitHub repo

While in general you can apply the output of git diff to another working tree, in this case you can't. The reason is that git diff shows that your submodule is dirty: that is, it has uncommitted changes. Since there's no way to represent these changes in the diff as it stands, any attempt to apply them will fail.

If you want to exclude these submodule changes, you can use git diff --ignore-submodules, which will ignore your submodules, whether modified or not. If you want to include them recursively, you can use git diff --submodule=diff. That patch won't update the submodule commits themselves, though it will change the submodule working tree, so it's not recommended.

Both of those should produce appliable patches.