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.

[naseschwarz]

First half is obvious, second half is its usage, not a description of .0

[peso]

I think usage is important in this case.

The .0 was really strange to me when I discovered it in code. After seeing that it was the index in the commit list, it made even less sense to me. It was not until I saw it being sorted in fn mark() that I figured why it was added - to make the list order stable to insertions and deletions, so I think the motivation for having it there is important.

I considered changing it to m̀arked: Vec<CommitId> (no pair) which would be easier to understand. The sorting would then have to look up the index using self.commits.get_index_of() but I expected somebody to argue that it would be some clock cycles slower.

I also wanted to keep the comment condensed - as few non-obvious parts as possible. The usage of .0 was the only thing not obvious to me.

If you prefer a more verbose comment that puts the "what" (description of content) before the "why" (description of function), how about this:

/// The marked commits.
/// .0 holds the commit index into self.items.items - used for ordering the list
/// .1 is the commit id of the marked commit