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.
echo % initialize remote repo with branches
hg init remote
cd remote
echo a > a
hg ci -Ama
hg branch br
hg ci -Amb
echo c > c
hg ci -Amc
hg log
cd ..
echo % try cloning -r branch
hg clone -rbr remote local1
hg -R local1 parents
echo % try cloning -rother clone#branch
hg clone -r0 remote#br local2
hg -R local2 parents
echo % try cloning -r1 clone#branch
hg clone -r1 remote#br local3
hg -R local3 parents