Doesn't respect submodule update=none strategy in .gitmodules

[sdroege]

See rust-lang/cargo#10717 for the corresponding fix in cargo.

When running cargo outdated on such a repository, it will still download all the git submodules while cargo update for example won't.

$ cargo update
    Updating git repository `https://github.com/gtk-rs/gtk-rs-core`
    Skipping git submodule `https://github.com/gtk-rs/gir` due to update strategy in .gitmodules
    Skipping git submodule `https://github.com/gtk-rs/gir-files` due to update strategy in .gitmodules
[...]

$ cargo outdated
  Parsing... current workspace
Resolving... current workspace
    Updating git submodule `https://github.com/gtk-rs/gir`
    Updating git submodule `https://github.com/gtk-rs/gir-files`