linecount: ensure we start a visible beginning of document

linecount: ensure we start a visible beginning of document

The document we are counting might be a 'view' and the early lines might
be clipped off.  So use doc:set-ref to move to the beginning as visible
from the focus.
Also simplify the code as two conditions each have exactly the same result.

Signed-off-by: NeilBrown <neil@brown.name>