sanitize-commit: combine $ln and $lineno variables

Use a single variable for the current line number of the commit message
and of the current file, in preparation for new checks which use this information.

Note that $lineno always counts from 1. Previously, $ln counted from 0 while
$lineno counted from 1.

Change-Id: I3a87d29df1378e304a27a7672630f0319dc863a2
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>