Experiment with a files-changed.json per PR (#161788)

This is an experiment to determine what the wall time of a (minimal)
"upload a JSON file describing files changed".

We are looking for this to take (low) single digit minutes, because if
we use this file (and expand it to become a build plan), it will block
almost _all_ other actions, and will likely get longer as we add more
complex logic and checks. Here's hoping!

dev/tools/bin/GITHUB_ACTIONS.md

@@ -0,0 +1 @@
+This is a stub-file that can be deleted.

dev/tools/bin/get_files_changed.dart

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+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:convert';
+import 'dart:io' as io;
+
+import 'package:args/args.dart';
+
+final ArgParser _argParser =
+    ArgParser()
+      ..addOption(
+        'since',
+        help: 'What previous SHA to compare the current git state to.',
+        defaultsTo: 'HEAD^',
+      )
+      ..addOption(
+        'output',
+        help: 'What format to output in.',
+        defaultsTo: io.stdout.hasTerminal ? 'text' : 'json',
+        allowed: <String>['text', 'json'],
+      );
+
+void main(List<String> args) async {
+  final ArgResults argResults = _argParser.parse(args);
+
+  // Get a list of files changed between this commit and the base SHA.
+  final List<String> filesChanged;
+  {
+    final List<String> args = <String>[
+      'diff',
+      '--name-only',
+      '--full-index',
+      argResults.option('since')!,
+    ];
+    final io.ProcessResult git = await io.Process.run('git', args);
+    if (git.exitCode != 0) {
+      io.stderr.writeln('$args failed (exit code: ${git.exitCode}):');
+      io.stderr.writeln(git.stdout);
+      io.stderr.writeln(git.stderr);
+      io.exitCode = 1;
+      return;
+    }
+
+    final String stdout = git.stdout as String;
+    filesChanged = const LineSplitter().convert(stdout);
+  }
+
+  // Output to stdout.
+  final _Output output = _Output.values.byName(argResults.option('output')!);
+  io.stdout.writeln(switch (output) {
+    _Output.text => filesChanged.join('\n'),
+    _Output.json => jsonEncode(filesChanged),
+  });
+}
+
+enum _Output { text, json }