bspatch: Fix arg names.

In last CL we introduced a bug where the old_filename was mixed with
the new_filename. This obvious bug didn't trigger any failure because
update_engine uses always the same name (/dev/fd/N) for both old and
new files since the files are applied in place, and also because bsdiff
doesn't have any unittest... yet.

BUG=None
TEST=sudo emerge bsdiff; tested bspatch manually.
TEST=cros flash an image with payload minor_version=2, which uses different old and new files.

Change-Id: I8b5715cd422612b9bbf7eaf3d4d3813c659fd10c