Ref T2222. This is the big one.

This migrates each DifferentialComment to one or more ApplicationTransactions (action, cc, reviewers, update, comment, inlines), and makes DifferentialComment a double-reader for ApplicationTransactions.

The migration is pretty straightforward:

• If a comment took an action not otherwise covered, it gets an "action" transaction. This is something like "epriestley abandoned this revision.".
• If a comment updated the diff, it gets an "updated diff" transaction. Very old transactions of this type may not have a diff ID (probably only at Facebook).
• If a comment added or removed reviewers, it gets a "changed reviewers" transaction.
• If a comment added CCs, it gets a "subscribers" transaction.
• If a comment added comment text, it gets a "comment" transaction.
• For each inline attached to a comment, we generate an "inline" transaction.

Most comments generate a small number of transactions, but a few generate a significant number.

At HEAD, the code is basically already doing this, so comments in the last day or two already obey these rules, roughly, and will all generate only one transaction (except inlines).

Because we've already preallocated PHIDs in the comment text table, we only need to write to the transaction table.

Specifically, they look like this:

{F112270}

I've migrated locally and put things through their paces, but it's hard to catch sketchy stuff locally because most of my test data is nonsense and bad migrations wouldn't necessarily look out of place.

I generally feel good about this change: it's not that big since we were able to separate a lot of pieces out of it, and it's pretty straightforward. That said, it's still one of the most scary/dangerous changes we've ever made.