Add exercise for extracting part of repo with history

[martinmosegaard]

Setup: A repo has grown too big and needs to split in two, whilst preserving history.
Solution: Use git subtree like this: https://ariya.io/2014/07/extracting-parts-of-git-repository-and-keeping-the-history

[RandomSort]

It is a good idea. Although the traceability of cause will be amiss ( unless you tag commits with previous commit shas? confusing! ).
Another solution could be to use filter-branch or patches to extract. Or orphaned branches.
Hmm. Problem is definitely interesting!