prevent fullscreen refresh when in single-line horiz scroll

If the full screen is scrolling horizontally, we must refresh the full
screen.  However, if doing single line scrolling, we only need to
refresh that line.

	modified:   src/filebuffer.rb
bugfix: single line horiz scroll was broken

Forgot to take into account @linefeed.

	modified:   src/filebuffer.rb
bugfix: handle being asked to open a directory

It was crashing, now it just returns an empty buffer.

	modified:   src/fileaccessor.rb