Add rollup=iffy

[Manishearth]

Overall when making rollups I roughly categorize PRs into four categories: never, always, iffy, and normal:

• Never PRs are marked rollup=never and should never be rolled up.
• Always PRs are either marked rollup=always or should be and contain changes that are unlikely to break tests.
• Iffy vs normal is a judgement call I make: normal ones are ones which are unlikely to cause issues, whereas iffy ones are ones which e.g. make changes to codegen, platform stuff, or the build system.

It would be nice if homu had an extra category for this and sorted the PRs appropriately.

When I make rollups, I try to include some iffy PRs, but I try to limit it.

[Manishearth]

Actually, while we're at this we should prioritize rollup=never>iffy>none>always