Fix submodule checkout substitution for POSIX shells

The substitution used in the submodule checkout command does not work when `/bin/sh` is a real POSIX shell (such as pdksh.) This change fixes the substitution so it will work with all shells.
`toplevel` and `path` are populated by git: http://git-scm.com/docs/git-submodule (see docs under `foreach`).