Add commands to go to the prev/next change in the list

When viewing a change, 'n' and 'p' will jump to the next and
previous changes (respectively) from the most recent list of changes
without pushing the current change on to the navigation history.

This streamlines the process when reviewing many changes in
sequence.

Change-Id: I31d4b30cd8d7bd6f5c5c4376033e43421fbe6c5b