Add --reverse option to replace-fork script (facebook#20249)

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.

scripts/merge-fork/replace-fork.js

@@ -8,10 +8,15 @@ const {promisify} = require('util');
 const glob = promisify(require('glob'));
 const {spawnSync} = require('child_process');
 const fs = require('fs');
+const minimist = require('minimist');
 
 const stat = promisify(fs.stat);
 const copyFile = promisify(fs.copyFile);
 
+const argv = minimist(process.argv.slice(2), {
+  boolean: ['reverse'],
+});
+
 async function main() {
   const oldFilenames = await glob('packages/react-reconciler/**/*.old.js');
   await Promise.all(oldFilenames.map(unforkFile));
@@ -42,7 +47,11 @@ async function unforkFile(oldFilename) {
     return;
   }
 
-  await copyFile(newFilename, oldFilename);
+  if (argv.reverse) {
+    await copyFile(oldFilename, newFilename);
+  } else {
+    await copyFile(newFilename, oldFilename);
+  }
 }
 
 main();