setup_revisions() calls diff_setup_done() before we can set default
value for output_format. Don't convert DIFF_FORMAT_NO_OUTPUT to 0 in
diff_setup_done(), it is useless and makes diff-tree believe no diff
format parameters were given and thus lets it reset output_format to
DIFF_FORMAT_RAW.
Signed-off-by: Timo Hirvonen <tihirvon@gmail.com> Signed-off-by: Junio C Hamano <junkio@cox.net>