Opt-in preference for checking out HEAD of submodule in Git

See discussions here: https://answers.atlassian.com/questions/154204/git-submodule-update-on-branch-switch

Basically need an opt-in option in the prefs on the Git tab to have an option whereby you can checkout the HEAD of a submodule regardless of what commit you're checking out on any branch. This is only available in Git any way so can't be done in Mercurial.