This feature is called "orthogonal edge routing" and is available in Graphviz versions from September 28, 2010 and newer. Use "graph [splines=ortho]"; see the Graphviz documentation.
digraph Orthogonal {
graph [label="Orthogonal edges", splines=ortho, nodesep=0.8]
node [shape=box]
a->{b c}
b->{d e}
c->{f g}
}