Sensible submodule configuration

[IoannisNezis]

This PR is ment to fix issue: #32
After running something like:
git submodule update --init --recursive
The submodules should conain remote changes and be on branch.

When i googled this issue i found this:
https://stackoverflow.com/questions/18770545/why-is-my-git-submodule-head-detached-from-master
And i also took a closer look at the Docu:
https://git-scm.com/book/en/v2/Git-Tools-Submodules

How i want to fix this:

submodule.<submodule-path>.update sets the update method.
This defaults to checkout.
"the commit recorded in the superproject will be checked out in the submodule on a detached HEAD"
I think the more sensible method is rebase:
"the current branch of the submodule will be rebased onto the commit recorded in the superproject"

submodule.<submoduke-path>.branch sets the remote branch for a submodule.
As far as i can tell this only has an effect during initialization... this is where i start to get confused.

There is a Problem with these settings... I think.
rebase rebases the **current ** branch of the submodule.
If i have a submodule checked out on a feature branch and i run git submodule updatethis would change the content of the feature branch and not the content of main.
This might be confusing and create chaos.

[pastewka]

Happy to work with rebases, but this does not make thing easier necessarily. It leads to cleaner repositories though. (We are using a rebase-only policy in another large project. While that works, there it leads to more effort rebasing diverging branches.)