Add --stash as alternative to --clean to update-checkout (#71178)

Use of `--clean` can lead to irreversible loss of uncommitted data. We still need to reset Swift project repositories to a clean state, but without deleting all in-progress changes. Passing `--stash` instead of (or in addition to) `--clean` will preserve uncommitted changes in stashes of corresponding repositories.

utils/update_checkout/update_checkout/update_checkout.py

@@ -148,7 +148,7 @@ def get_branch_for_repo(config, repo_name, scheme_name, scheme_map,
 
 def update_single_repository(pool_args):
  source_root, config, repo_name, scheme_name, scheme_map, tag, timestamp, \
- reset_to_remote, should_clean, cross_repos_pr = pool_args
+ reset_to_remote, should_clean, should_stash, cross_repos_pr = pool_args
  repo_path = os.path.join(source_root, repo_name)
  if not os.path.isdir(repo_path) or os.path.islink(repo_path):
  return
@@ -171,9 +171,13 @@ def update_single_repository(pool_args):
  checkout_target)
 
  # The clean option restores a repository to pristine condition.
- if should_clean:
- shell.run(['git', 'clean', '-fdx'],
- echo=True, prefix=prefix)
+ if should_clean or should_stash:
+ if should_stash:
+ shell.run(['git', 'stash'],
+ echo=True, prefix=prefix)
+ elif should_clean:
+ shell.run(['git', 'clean', '-fdx'],
+ echo=True, prefix=prefix)
  shell.run(['git', 'submodule', 'foreach', '--recursive',
  'git', 'clean', '-fdx'],
  echo=True, prefix=prefix)
@@ -343,6 +347,7 @@ def update_all_repositories(args, config, scheme_name, scheme_map, cross_repos_p
  timestamp,
  args.reset_to_remote,
  args.clean,
+ args.stash,
  cross_repos_pr]
  pool_args.append(my_args)
 
@@ -605,7 +610,11 @@ def main():
  action='store_true')
  parser.add_argument(
  '--clean',
- help='Clean unrelated files from each repository.',
+ help='Clean untracked files from each repository.',
+ action='store_true')
+ parser.add_argument(
+ '--stash',
+ help='Stash untracked files from each repository.',
  action='store_true')
  parser.add_argument(
  "--config",