Revise version control module

[JaroCamphuijsen]

• [ ] exercise version_control_exercise1.md could use some rethinking about
• [ ] exercise version_control_exercise2.md asks learners to name various version control systems and tools. We might want to think of a better way to let them explore version control tools. The most used tool is most definitely git, the others are niche.
• [ ] ...?

[JaroCamphuijsen]

117 is the original, github hickup made two