Arbitrary symlink creation via cached prefix reuse

Link following in gix-fs before 0.21.1 allows local users to create attacker-controlled symlinks outside a checked-out worktree. Stack::make_relative_path_current reuses a cached leaf as the next path prefix and calls push_directory instead of push, bypassing the metadata and collision checks before creating a Unix symlink. Exploitation requires checkout of a malicious tree containing duplicate symlink and directory entries, and writes are limited to existing directories writable by the checkout user.