tests/test-diffdir.out

Move commands.forget over to using new walk code.

With no names, it now recursively forgets everything, as is the default
behaviour of other commands.  And prints the names of all files it
hasn't specifically been told to forget.

diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123