Add keyboard bindings n/p for all change lists to paginate

This makes it easier to browse through a very long list of changes,
as you can use the keyboard keys n to move to the next page and p
to move to the prior, along with the existing j/k/o to move around
within the page, and open a record.

Signed-off-by: Shawn O. Pearce <sop@google.com>
3 files changed