Display branch label in right-hand (fixed) panel on a merge request

Description

(Include problem, use cases, benefits, and/or goals)

Problem: When reviewing a merge request, if I want to navigate to the branch, or just get the name of the branch (e.g. to check out locally and make some changes), I have to scroll back to the top of the page (but not quite the top; it's below the description, which can be long with our templates). Then when done, I scroll back to where I was in the review (so, I mentally make a note of where the scrollbar was). This introduces unnecessary friction. If the branch name was shown in the right-hand-side bar, it'd be always visible/available.

Proposal

Add the branch name/link to the right-hand bar on merge requests.