Made makemessages leave `'%%'` sequences untouched when extracting...

Made makemessages leave `'%%'` sequences untouched when extracting translatable literals from blocktrans template tags.

This makes it consistent with behavior introduced when fixing #11240 in
processing of literal passed to the trans tag to avoid double escaping
(i.e. `'%%%%'` sequences in resulting PO files.)

Also, cleaned up tests changes from r17190 (removed commented out code and
implemented compatibility with Python 2.5.)

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17192 bcc190cf-cafb-0310-a4f2-bffc1f526a37