tempfile: prefer is_tempfile_active to bare access

tempfile: prefer is_tempfile_active to bare access

The tempfile code keeps an "active" flag, and we have a
number of assertions to make sure that the objects are being
used in the right order. Most of these directly check
"active" rather than using the is_tempfile_active()
accessor.

Let's prefer using the accessor, in preparation for it
growing more complicated logic (like checking for NULL).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>