Fixed UI bugs with sticky diff header

What does this MR do?

Fixes a few UI bugs with the sticky diff file header.

1. Fixes the margin at the top on the changes tab.
2. Fixes the margin at the top on the commit tab.
3. Fixes a bug on mobile Safari with the performance bar enabled.
4. Fixes the changes bar on commits not sticking correctly.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54503

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.