merge-recursive: take advantage of hardcoded empty tree

When this code was first written (v1.4.3-rc1~174^2~4, merge-recur: if
there is no common ancestor, fake empty one, 2006-08-09), everyone
needing a fake empty tree had to make her own, but ever since
v1.5.5-rc0~180^2~1 (2008-02-13), the object lookup machinery provides
a ready-made one.  Use it.

This is just a simplification, though it also fixes a small leak
(since the tree in the virtual common ancestor commit is never freed).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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