Test that symlinks to directories are usable

At least as `std::fs::metdata` currently works on Windows, this
entails that symlinks to directories on Windows are created as
directory symlinks rather than as file symlinks (since file
symlinks cannot always be automatically dereferenced, and accessing
metadata via the `stat`-like `std::fs::metadata` function is one
situation where that cannot be done).

gix-worktree-state/tests/fixtures/make_dir_symlink.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -eu -o pipefail
+
+git init -q
+
+target_oid=$(echo -n "." | git hash-object -w --stdin)
+
+git update-index --index-info <<EOF
+120000 $target_oid	symlink
+EOF
+
+git commit -m "symlink in index, points to directory"

gix-worktree-state/tests/state/checkout.rs

@@ -267,6 +267,34 @@ fn symlinks_become_files_if_disabled() -> crate::Result {
     Ok(())
 }
 
+#[test]
+fn symlinks_to_directories_are_usable() -> crate::Result {
+    let opts = opts_from_probe();
+    if !opts.fs.symlink {
+        eprintln!("Skipping directory symlink test on filesystem that doesn't support it");
+        return Ok(());
+    }
+
+    let (_source_tree, destination, _index, outcome) =
+        checkout_index_in_tmp_dir(opts.clone(), "make_dir_symlink", None)?;
+    let worktree_files = dir_structure(&destination);
+    let worktree_files_stripped = stripped_prefix(&destination, &worktree_files);
+
+    assert_eq!(worktree_files_stripped, paths(["symlink"]));
+    let symlink_path = &worktree_files[0];
+    assert!(symlink_path
+        .symlink_metadata()
+        .expect("symlink is on disk")
+        .is_symlink());
+    assert!(symlink_path
+        .metadata()
+        .expect("metadata accessible through symlink")
+        .is_dir());
+    assert_eq!(std::fs::read_link(symlink_path)?, Path::new("."));
+    assert!(outcome.collisions.is_empty());
+    Ok(())
+}
+
 #[test]
 fn dangling_symlinks_can_be_created() -> crate::Result {
     let opts = opts_from_probe();