t4013: add tests for diff/log family output options.

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/t/t4013/diff.diff-tree_initial b/t/t4013/diff.diff-tree_initial
new file mode 100644
index 0000000..b49fc53
--- /dev/null
+++ b/t/t4013/diff.diff-tree_initial
@@ -0,0 +1,2 @@
+$ git diff-tree initial
+$