contributing: move "git rebase" advice to bottom

The idea is to keep the guidelines at the top light-weight and easy to
read. The more heavy Git commands are then moved to the bottom part.
contributing: move "git rebase -i" text to bottom

Separating the detailed commands from the guideline above allows us to
expand on the commands without fearing that we obscure the message in
the guidelines.
contributing: move "git commit --amend" text to bottom

This also makes the guideline above more clear.