MQ: break up a 'qrefresh'

[John W]

I know how to do what I want, but before I write an extension for it,
I figured I'd ask if there's some better and/or built-in way to do it.

It's a pretty simple situation:

1. You have a patch applied (call it P1), and it modifies the file 'foo'.
2. You make some updates to 'foo' and run 'qrefresh'. Now P1 is
modified (call it P1*).
3. You decide that actually, those changes should be a separate patch (P2).

How best to extract those changes to P1 into a new patch, P2?

In other words: You have P1*, and you want P1, P2

If the MQ patches have their own repo, this is doable with some
moderate shenanigans - you need to create one head with P1 and another
with P1*, do a diff between them, save that diff as P2, etc.

Is there a more straightforward way to accomplish this in Mercurial?
it seems like a fairly common occurrence, so I imagined there might be.

Thanks
-John