When doing typeahead searching, hitting the up/down arrow keys lets you cycle
between the files that start with the prefix you typed.  This is useful when you
have both files and folders starting with the prefix; "foo" in this example:

  bar/
  foo/
  foofum/
  fooqux/
  gnoghurt/
  ... lots of folders here ...
  a.txt
  bee.txt
  foo.txt
  foobar.jpg
  winkwink.jpg

In this example, hitting the up/down keys after typing "foo" lets you jump from
the folders to the (distant) "foo-" files easily.  However, there's no obvious
way of how to end this cycling action, to let you use the arrow keys normally again.

Also, if you have a single file with the prefix, hitting up/down appears to do
nothing because they cycle among the same file.

Proposal:

- If there is only a single file with the prefix, no cycling happens.  Hitting
up/down gets you out of interactive search immediately, and selects the
previous/next file in the list, respectively.

- If there is more than one file with the prefix, hitting up/down cycles among
those files.  But if you hit the topmost file in that subset and hit Up,
interactive search ends.  If you hit the bottommost file in the subset and hit
Down, the search also ends.  These would take you to the files immediately
above/below the files with the prefix, respectively.