mergetools/p4merge: Honor $TMPDIR for the /dev/null placeholder

mergetools/p4merge: Honor $TMPDIR for the /dev/null placeholder

Use $TMPDIR when creating the /dev/null placeholder for p4merge.
This prevents users from finding a seemingly random untracked file
in their worktree.

This is different than what mergetool does with $LOCAL and
$REMOTE because those files exist to aid users when resolving
merges.  p4merge's /dev/null placeholder is not helpful in that
situation so it is sensible to keep it out of the worktree.

Reported-by: Jeremy Morton <admin@game-point.net>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>