tests/test-diffdir

Fix up parent display in logging

For changesets with a first parent nullid, but not rev 0, we ought to
display a null parent.

#!/bin/sh

hg init
touch a
hg add a
hg ci -m "a" -d "1000000 0"

echo 123 > b
hg add b
hg diff --nodates

hg diff --nodates -r tip

echo foo > a
hg diff --nodates