git format-patch doesn't exclude merged hunks

So I was using `git format-patch` to generate patches for
consumption by patch (as part of an RPM build),
and one of the patches was failing as part of it was
already applied.

It seems like format-patch should exclude bits it's
merge previously, at least as an option.

For reference the two commits in question are:
https://github.com/openstack/nova/commit/7028d66
https://github.com/openstack/nova/commit/26dc6b7
Notice how both make the same change to Authors.

cheers,
Pádraig.