Redesign commit header

[Uncommon]

The commit header turns out to be hard to read at a glance. The parent links stand out more than the first line of the summary, and at the default split size the summary is sometimes totally hidden.

Maybe have a two-column layout with summary on one side and metadata on the other.

This is also a good opportunity to make the parent links appear in a consistent place for easier navigation. Now they move depending on whether the author and committer are separate.