`gh repo sync` does not update submodules in local repo

Describe the bug

When local repo is used as the destination repo, gh repo sync does not update submodules. --recurse-submodules

Same issue in other gh subcommands:

• Both gh repo clone and gh repo fork supports passing additional Git flags after --.
  • https://github.com/cli/cli/issues/4616
  • https://github.com/cli/cli/issues/6152
• gh pr checkout supports flag --recurse-submodules.
  • https://github.com/cli/cli/issues/368

Affected version

gh version 2.96.0 (2026-07-02) https://github.com/cli/cli/releases/tag/v2.96.0

Steps to reproduce the behavior

Using https://github.com/Witiko/expltools as an example. It sets two submodules.

# preparations
$ git clone --revision=a13cc24554adf90e3a6f4a82a8b8528cadb2f412 --depth=1 [email protected]:Witiko/expltools.git
$ cd expltools
$ git switch -c main
$ git submodule update --init --recursive --depth=1

$ gh repo sync --branch main
✓ Synced the "main" branch from "Witiko/expltools" to local repository
$ git status
On branch main
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   third-party/latex3 (new commits)

no changes added to commit (use "git add" and/or "git commit -a")

Expected vs actual behavior

Expected: gh repo sync updates submodules and leaves a clean working tree, either automatically or by passing some extra (Git) flag.

Actual: If the sync-ed commits contains submodule updates, gh repo sync leaves submodule directori(es) dirty, see the git status output shown above.