I found myself with a repo containing a superset of the patches requried
for a given dpatch. I'd like to obliterate all patches not in the
context of the dpatch.

I also wanted to pull in any patches that my repo didn't have, that were
in the context (without having to manually pull and select patches).

Reason for these weird wishes:

I'm reviewing a couple of patches that I know have similar contexts, if
not the same. I've already reviewed one patch, using get --context from
my up-to-date local screened repo.
However, I don't want to have to mess about with another get --context
so I'd like to be able to tweak my review repo more easily.