Cloning of submodule with --depth should follow the branch option

Hi,

I have an issue with git 2.4.3 when trying to limit the amount of data
fetched when cloning submodules. The --depth option is useful, but
when .gitmodules specifies a non-default branch, the submodule
repository clone invocation (which is a single-branch clone due to
--depth) fetches master regardless, and then checkout fails due to a
nonexistent tree reference. I think it would be sensible in this case
to pass --branch to the submodule clone command.

Here's a repository to exercise: https://github.com/gi-rust/gir

Best regards,
  Mikhail