gitoxide: Symlink prefix-reuse allows worktree escape during checkout

Summary

gitoxide is an implementation of git written in Rust. Prior to 0.21.1, a malicious tree can be constructed that will, when checked out with gitoxide, permit writing an attacker-controlled symlink into any existing directory the user has write access to. During checkout, all symlink index entries are deferred and created after regular files using a single shared gix_worktree::Stack. Internally, this uses a gix_fs::Stack. gix_fs::Stack::make_relative_path_current() caches validated path prefixes: when the previously-processed leaf component exactly matches the leading component(s) of the next path, the leaf-to-directory transition at gix-fs/src/stack.rs invokes only delegate.push_directory(), never delegate.push(). In gix_worktree::stack::delegate::StackDelegate, when the state member is State::CreateDirectoryAndAttributesStack, Attributes::push_directory() only loads attributes (from the ODB, in the clone case), and does not perform any other checks. The on-disk symlink_metadata() check and unlink-on-collision live in StackDelegate::push()'s invocation of create_leading_directory(), which is therefore bypassed for the cached prefix. The final symlink is created with plain std::os::unix::fs::symlink, which follows symlinks in parent directories. Therefore, it's possible to provide a tree with duplicate symlink and directory entries that exploits this. This vulnerability is fixed in 0.21.1.