create non-existent files in edit-sources with 644 instead of 640

create non-existent files in edit-sources with 644 instead of 640

If the sources file we want to edit doesn't exist yet GetLock will
create it with 640, which for a generic lockfile might be okay, but as
this is a sources file more relaxed permissions are in order – and
actually required as it wont be readable for unprivileged users causing
warnings/errors in apt calls.

Reported-By: J. Theede (musca) on IRC