replaced USERNAME and TESTREPO with variables in thw-git

This responds to difficulties at 2013-11-14-woodshole in which the USERNAME and TESTREPO were not find-and-replaced in advance of the git lesson.  It took time for students to discover that TESTREPO was a placeholder, and this got in the way of following along.

This sets the default  TESTREPO to 2013-12-25-euphoric  and USERNAME is instructor.