tests/test-586

graphlog: split the actual DAG grapher out into a separate method

This allows extensions like pbranch to use asciigraph() to graph dependencies
for patch branches, but could be used for basically any DAG.

#!/bin/sh
# a test for issue586

hg init a
cd a
echo a > a
hg ci -Ama

hg init ../b
cd ../b
echo b > b
hg ci -Amb

hg pull -f ../a
hg merge
hg rm -f a
hg ci -Amc

hg st -A