worktree: add --recurse-submodules support to git worktree add

[Ergus]

"git worktree add" leaves submodules uninitialised in the new working
tree. This series adds --recurse-submodules to initialise them
automatically, using the same per-worktree-gitdir model that linked
worktrees already use for the superproject.

The key design choice is where the per-worktree submodule gitdir lives.
An earlier iteration (v1) nested it under the superproject's own
worktrees/ tree:

$GIT_COMMON_DIR/worktrees/<wt-id>/modules/<name>/

Junio pointed out that this placement is invisible to the shared
submodule repository, so "git gc" on the shared repo can prune commits
reachable only from a per-worktree HEAD. This series follows the
approach he suggested instead: treat the absorbed submodule repo as a
bare-like repo and create proper linked worktrees off it:

$GIT_COMMON_DIR/modules/<name>/worktrees/<wt-id>/

Because this path lives inside the shared repo's own worktrees/ tree,
"git gc" finds it via get_worktrees() and protects per-worktree objects.

The series is structured as follows:

1/4 submodule: place per-worktree gitdir under shared repo
      Teaches submodule_name_to_gitdir() to return the new path in
      linked worktrees and fixes validate_submodule_legacy_git_dir()
      to handle the new path shape.

2/4 submodule--helper: init per-worktree submodule gitdir
      Wires up clone_submodule() to create the per-worktree gitdir at
      the new location, with auto-absorb of legacy in-tree gitdirs and
      a guard in ensure_core_worktree() to prevent overwriting the
      shared repo's core.worktree.

3/4 worktree: add --recurse-submodules to worktree add
      Adds the flag to "worktree add" and teaches "worktree remove" to
      clean up per-worktree submodule gitdirs explicitly (since they
      no longer nest under the worktree's own gitdir).  Includes
      documentation.

4/4 t2403, t2405: test per-worktree submodule gitdir model
      Tests covering initialisation, removal, mixed/legacy layouts,
      nested submodule paths, and the auto-absorb code path.

[gitgitgadget]

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