When enabled, replaces new fork with old fork.
I've done this several times by manually editing the script file, so
seems useful enough to add an option.
* Pass extra CLI args through to ESLint
These now work:
```
yarn run lint --fix
yarn run linc --fix
```
* Autofix imports when running replace-fork
We have a custom ESLint rule that can autofix cross-fork imports.
Usually, after running the `replace-fork` script, you need to run
`yarn lint --fix` to fix the imports.
This combines the two steps into one.
* Add autofix to cross-fork lint rule
* replace-fork: Replaces old fork contents with new
For each file in the new fork, copies the contents into the
corresponding file of the old fork, replacing what was already there.
In contrast to merge-fork, which performs a three-way merge.
* Replace old fork contents with new fork
First I ran `yarn replace-fork`.
Then I ran `yarn lint` with autofix enabled. There's currently no way to
do that from the command line (we should fix that), so I had to edit the
lint script file.
* Manual fix-ups
Removes dead branches, removes prefixes from internal fields. Stuff
like that.
* Fix DevTools tests
DevTools tests only run against the old fork, which is why I didn't
catch these earlier.
There is one test that is still failing. I'm fairly certain it's related
to the layout of the Suspense fiber: we no longer conditionally wrap the
primary children. They are always wrapped in an extra fiber.
Since this has been running in www for weeks without major issues, I'll
defer fixing the remaining test to a follow up.
