contrib/git-viz/git-diff-tree

Use nullrev+count instead of count-1 in a newly introduced expression.
Before that repo.changelog.tip() was used, but this yields a node.

Of course many more places should be changed like this, this was just done
as an example.

#!/bin/sh

op=`basename $0 | sed -e 's/^git-//'`
exec hgit $op "$@"