How do we review changes made with coccinelle?

Ævar recently sent a series that made pretty extensive use of coccinelle to remove a lot of the_repository [1], but then we ended up in this weird spot where even though several reviewers thought the code changes looked good, we weren't sure about giving Reviewed-By because we didn't know coccinelle or how to review it.

I'm pretty sure that the series could have been merged without a hitch if reviewers knew what to do about coccinelle. I expect more of these to appear as a result of the libification work, so it's probably a good time for us to figure out some norms about coccinelle :)

Perhaps we could start the discussion by sharing thoughts on the following questions, which I'll summarize in a change to contrib/coccinelle/README (where we can do final bikeshedding):

- Is it okay to give Reviewed-By on the basis of _just_ the in-tree
  changes and ignore the .cocci patch?

  - If not, what should reviewers look for in .cocci? Do we have a
    style?

- When do we introduce .pending.cocci vs .cocci?

- What do we do with .cocci after they've been applied?

- Do we care about new patches slowing down coccicheck?

Relevant threads
- How to learn cocci: https://lore.kernel.org/git/230326.86edpcw0yh.gmgdl@evledraar.gmail.com/ 
- https://lore.kernel.org/git/230328.86a5zxw31u.gmgdl@evledraar.gmail.com/
- https://lore.kernel.org/git/230326.86ileow1fu.gmgdl@evledraar.gmail.com/

[1] https://lore.kernel.org/git/cover-v2-00.17-00000000000-20230328T110946Z-avarab@gmail.com/