Script for syncing changes between forks (#18550)

Adds command `yarn merge-fork`.

```sh
yarn merge-fork --base-dir=packages/react-reconciler/src ReactFiberWorkLoop
```

This will take all the changes in `ReactFiberWorkLoop.new.js` and apply
them to `ReactFiberWorkLoop.old.js`.

You can merge multiple modules at a time:

```sh
yarn merge-fork \
  --base-dir=packages/react-reconciler/src \
  ReactFiberWorkLoop \
  ReactFiberBeginWork \
  ReactFiberCompleteWork \
  ReactFiberCommitWork
```

You can provide explicit "old" and "new" file names. This only works
for one module at a time:

```sh
yarn merge-fork \
  --base-dir=packages/react-reconciler/src \
  --old=ReactFiberExpirationTime.js \
  --new=ReactFiberLane.js
```

The default is to merge changes from the new module to the old one. To
merge changes in the opposite direction, use `--reverse`.

```sh
yarn merge-fork \
  --reverse \
  --base-dir=packages/react-reconciler/src \
  ReactFiberWorkLoop
```

By default, the changes are compared to HEAD. You can use `--base-ref`
to compare to any rev. For example, while working on a PR, you might
make multiple commits to the new fork before you're ready to backport
them to the old one. In that case, you want to compare to the merge
base of your PR branch:

```sh
yarn merge-fork \
  --base-ref=$(git merge-base HEAD origin/master)
  --base-dir=packages/react-reconciler/src \
  ReactFiberWorkLoop
```