RFC: have a config option for running submodule update after checkout <revision>

When checking out a new revision in a repo with submodules, one of the first commands after checkout is `git submodule update` to make sure the submodules are updated.

I propose to have a switch which when turned on automatically runs `git submodule update` for you instead of implementing proper submodule support in checkout for example.

advantages:
* users are already used to the quirks of `submodule update`
* a lot of config options exist for the behavior of `submodule update`

disadvantages:
* it's not `done right`, i.e. dealing with gitlinks in read-tree/unpack-tree
   but just another command chained after checkout, so we would do that
   for pull too eventually?

What do you think?

Thanks, Stefan