It's not currently convenient to checkout a SmashPig Gerrit patch inside the payments-wiki composer vendor directory when reviewing a patch that depends on an unmerged SmashPig update.

We currently do this in one of two ways: either with risky docker overrides that has caught us out in the past or we do it via manually delete/reclone/checkout/install the patch at /vendor/Wikimedia/smash-pig.

Let's script it up to make this easier.