Distinguish merge commits in the active commit view

[xordanblu]

Refs gogs#2696

Prepared for the funded IssueHunt task.

What changed

• Style the parent-count metadata as a muted badge only when the active React commit view contains at least two adjacent parent links.
• Leave ordinary one-parent commits unchanged.
• Scope the selector to the commit-diff page.

Why it identifies merge commits

Git represents a merge commit with more than one parent. The current React page already renders each parent SHA as an adjacent commit link. The selector activates only when the parent-count span is followed by at least two such links.

Validation

• Branch based on current main.
• Only web/src/index.css changed: 15 additions.
• Web workflow: passed.
• Shell workflow: passed.
• Formatting, lint, and production build all pass.