Add rebase-multiple-commits exercise

[BaoCaiH]

Add exercise suggested in issue #354

[sofusalbertsen]

Hey there, thanks for your PR!
I will try to get some eyes on it this week 👀

[JKrag]

Thanks for the reminder @sofusalbertsen. I had already taken a quick look at this 2 weeks ago when I was in Helsinki, and was testing it out and testing some small changes last week, but then I ran of of time. will try and get back to it tomorrow.

[JKrag]

Hi Bao. Thanks again for the PR.
The exercise works fine and illustrates some valid points and differences between the two scenarios.

I think I had a few ideas for minor improvements, when I looked at it a few weeks ago - something about changing the actual content of the files to make it more obvious what was before and after, helping with the conflict resolution, but maybe I am just overthinking it.
I think I will just merge it in as is, and try it out on some training participants. ;-)