revision: convert to using diff_tree_sha1()

revision: convert to using diff_tree_sha1()

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>