For merge requests, show metadata of individual commits and the overall combined diff together.

Filing this enhancement proposal as requested by @GitLab on Twitter:

When reviewing a merge request, it would be great to have the individual commits listed with their metadata (commit message, ID, author, timestamp) above the combined diff for the overall merge request, all on the same page.

This would make it possible to see all of the commentary about the individual changes as given in the commit messages, while still reviewing the merge request as the total overall change it is. One could still visit any individual commit to see its particular diff, of course, but in practice that's rare -- one usually wants to review the commit messages while looking at the combined textual diff. Right now one must switch between two contexts ("Commits" vs "Changes") to do that.

For example, take merge request 22036. It contains two commits. Right now, if I click on "Changes", I can see the diff, but I can't see what the author said about the changes -- I have to click over on "Changes" for that, but now I can't see the diff.

Essentially, I'm proposing that the "Commits" view go away, and that the "Changes" view start out with the list of commits (formatted the same way they are now in the "Commits" view, including the expandable "..." button to see the whole message for that commit), followed by the Changes view as it currently stands. No information is lost, but the merge request is presented in a more review-friendly way.

This is related to, but not the same as, issue gitlab-ce#49685.