Avoid exposing internal tuple in CommitList.marked

[peso]

This Pull Request is a minor cleanup

It changes the following:

• Use a different function to avoid exposing the tuple used to sort commit id in CommitList.marked

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

[naseschwarz]

Thank you for your PR. I have one item to pick on - if we add comments, they should be accurate.

[extrawurst]

lets fix the comment and get it merged