Hide comment button if on diff HEAD

What does this MR do?

Hides the comment button if the current diff is compared to "HEAD". This is a temporary change and will be reverted when comments are supported on HEAD diffs.

NOTE: To test simply load an MR with diff_head=true as a querystring. The comment bubbles should not show up on hover.

Relates to: !23157 (merged) #27008 (closed)

Screenshots

Notice that the hover state does not show a comment bubble.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Style guides