Since diff_tree_sha1() can now accept empty trees via NULL sha1, we
could just call it without manually reading trees into tree_desc and
duplicating code.
Besides, that
if (!tree)
return 0;
looked suspect - we were saying an invalid tree != empty tree, but maybe it is
better to just say the tree is invalid here, which is what diff_tree_sha1()
does for such case.
Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru> Signed-off-by: Junio C Hamano <gitster@pobox.com>