Navigating files in merge request diff

Problem

• Navigating between files in a merge request diff is fairly difficult, especially one with many files, example: gitlab-com/www-gitlab-com!4780 (diffs).
• How to make better?

Proposed solution

• Some type of file tree navigation so you can jump quickly to different files on the page by clicking on some UI, perhaps toward the side of the page.

Design

• The Showing X changed files with Y additions... (Screen 01) will be fixed on the top of the screen like tabs bar (Screen 02) after scrolling down.
• The bar includes:
  • X changed files
  • +112 -82 to indicate the amount of additions and deletions.
• After clicking X changed files, a dropdown will appear, and then the user can link to different files.

• There is +10 -5 to indicate the additions and deletions in the file.
• Users can also search the file in the dropdown.

Screen 01	Screen 02	Screen 03

( "in-between" icon to indicate a file was changed will be redesigned in #32894 (closed))