Workaround repo.submodules() to deal with unborn repositories.

This fix will come from `gitoxide`, but until that's released we
handle it here.

Doing so is fine becuase of the following.
The order in which submodule files are tried is this:

- worktree
- index
- HEAD:.gitmodules

If the `HEAD^{commit}` can't be read, this means there is definitely
no gitmodules file there. And since it's the last stage, there
is nothing we'd fail to discover.

Author: Byron

src/cargo/ops/cargo_package/vcs.rs

@@ -335,7 +335,16 @@ fn status_submodules(
     dirty_files: &mut Vec<PathBuf>,
     dirty_files_outside_of_package_root: &mut Vec<PathBuf>,
 ) -> CargoResult<()> {
-    let Some(submodules) = repo.submodules()? else {
+    let Some(submodules) = repo.submodules().or_else(|err| {
+        // TODO(@Byron): remove this `or_else()` once `submodules()` fixed in `gitoxide`,
+        //               and the release is available.
+        if matches!(err, gix::submodule::modules::Error::FindHeadCommit(_)) {
+            Ok(None)
+        } else {
+            Err(err)
+        }
+    })?
+    else {
         return Ok(());
     };
     for submodule in submodules {