Fixes the blueslip error on "i" in empty narrow.
Also removes a then-uncessary check from do_narrow_action as suggested
by acrefoot.
(imported from commit 10b1f702b535b4eef54e500ccef93b6a5280e953)
This restores the feature that hitting down when you are at the
last message automatically centers the last message. This is not
a pure revert, because some additional code now goes through
navigate.down.
(imported from commit 2db1f247692ba068613a2d6c93f18ca7c13a16b8)
Kevin Mehall
Steve Howell
Jeff Arnold
Tim Abbott