Minor improvements to discussion of rebasing.

[buchs]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

Context

No particular issues are related to these changes. I have never had much success with rebase (obviously, I merged your main branch into this one!). As I was reading the ProGit book on the subject for the nth time, I realized that the language is misleading as are the diagrams. Here are some minor and early improvements IMO.