Recursive submodule checkout

If I'd like to check out an older commit, lazygit only checks out that commit in the main repo, not the related submodules. This creates also a problem if the user clones a repo for the first time. I usually get a false idea that I haven't done something wrong (because submodules are marked as modified) but it is because they are not in sync with the current commit.

It would be good if we could have checked out those submodules recursively. Of course, they could be dirty, then an error message could be shown to the user about it.