fix: worktrees of submodules now know their correct worktree

Previously they would use a very incorrect worktree which would cause
the status to be calculated very wrongly.

gix/tests/fixtures/generated-archives/.gitignore

@@ -7,3 +7,4 @@
 /make_core_worktree_repo.tar
 /make_signatures_repo.tar
 /make_diff_repos.tar
+/make_submodule_with_worktree.tar

gix/tests/fixtures/make_submodule_with_worktree.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+set -eu -o pipefail
+
+git init -q module1
+(cd module1
+  touch this
+  git add . && git commit -q -m c1
+)
+
+git init submodule-with-extra-worktree-host
+(cd submodule-with-extra-worktree-host
+  git submodule add ../module1 m1
+  (cd m1
+    git worktree add ../../worktree-of-submodule
+  )
+)

gix/tests/gix/submodule.rs

@@ -11,6 +11,7 @@ mod open {
     use gix::submodule;
 
     use crate::submodule::repo;
+    use crate::util::named_subrepo_opts;
 
     #[test]
     fn various() -> crate::Result {
@@ -341,6 +342,24 @@ mod open {
         Ok(())
     }
 
+    #[test]
+    fn submodule_worktrees() -> crate::Result {
+        let sm_repo = named_subrepo_opts(
+            "make_submodule_with_worktree.sh",
+            "worktree-of-submodule",
+            gix::open::Options::isolated(),
+        )?;
+        let wd = sm_repo.work_dir().expect("workdir is present");
+        assert!(sm_repo.rev_parse_single(":this").is_ok(), "the file is in the submodule");
+        assert!(
+            wd.join("this").is_file(),
+            "The submodule itself has the file, so it should be in the worktree"
+        );
+
+        assert_eq!(sm_repo.worktrees()?.len(), 2, "The main worktree and the extra one");
+        Ok(())
+    }
+
     #[test]
     fn old_form() -> crate::Result {
         for name in ["old-form-invalid-worktree-path", "old-form"] {