[infra] Make worktrees work (again?)

History:

* Back in (pre June) 2018 worktrees in git just worked.
* https://dart-review.googlesource.com/c/sdk/+/48091 made it not work.
* https://github.com/dart-lang/sdk/issues/33619 was filed.
* https://dart-review.googlesource.com/c/sdk/+/127485 was created to
  work around the problem.

I don't know when it happened, but I can't make the above workaround
work (at least not anymore) --- adding a `default_git_folder` to my
`args.gn` has no effect.

This CL builds on it and automatically sets the correct path based on
a call to `git rev-parse --resolve-git-dir .git` which gives the
correct dir for both worktrees and non-worktrees.

Change-Id: I4dcf0445a44348621752fb88d4dab4c99c6ddd1d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/190522
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

sdk_args.gni

@@ -18,7 +18,10 @@ declare_args() {
   #
   # to out/ReleaseX64/args.gn. The path above can be extracted from the `.git`
   # file under the git worktree folder.
-  default_git_folder = "$_dart_root/.git"
+  # The script run here should take care of everything automatically though.
+  default_git_folder = exec_script("$_dart_root/tools/get_dot_git_folder.py",
+                                   [ rebase_path("$_dart_root/.git") ],
+                                   "trim string")
 
   # Whether to enable the SDK hash check that will prevent loading a kernel
   # into a VM which was built with a different SDK.

tools/get_dot_git_folder.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python
+# Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+#
+# This python script is a wrapper around `git rev-parse --resolve-git-dir`.
+# This is used for the build system to work with git worktrees.
+
+import sys
+import subprocess
+
+
+def main():
+    if len(sys.argv) != 2:
+        raise Exception('Expects exactly 1 argument.')
+
+    return subprocess.call(
+        ['git', 'rev-parse', '--resolve-git-dir', sys.argv[1]])
+
+
+if __name__ == '__main__':
+    sys.exit(main())