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.
Cc: Thomas Rast <tr@thomasrast.ch> Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru> Signed-off-by: Junio C Hamano <gitster@pobox.com>