magit-checkout-branch-at-point: remove unnecessary function

Instead add a `branch' case to `magit-visit-item'.
magit-insert-rebase-sequence: new function

I intend to further improve this.
magit-insert-unpulled-or-recent-commits: new function

Do not add it to the default value of `magit-status-sections-hook',
as I intend to make bigger changes to this and other log sections.