Allow JSON encoded environment diffs to be produced encoded in utf-8

On the one hand I could have encoded the miasmod diffs in cp1252 to
match what the game uses, but I think that more text editors will
default to utf-8, and I can't figure out the proper incantation to get
vim to treat the file as encoded in cp1252, so... utf-8 it is.
data.py: Don't print diff with --apply-diff