Reduce visual stacking and bars in merge request changes tab

Problem to solve

Reviewing the code changes introduced by a merge request is the primary purpose of the interface, however, the interface currently shows little discernment in which information to show when for the different tasks being performed in the merge request interface.

In comparison to our peers, GitLab's interface has more buttons and less focus.

Further details

Iteration based on #31300 (closed), and follow up to #33813 (closed).

Proposal

Improve visual balance of the page by having few bars stacked on bars

Links / references