graphmod: refactor state handling
Move ASCII graph state to a dictionary, to clarify what is being tracked. Move
the 'seen' state (tracking currently active edges) into this structure.
#require test-repo
$ cd "$TESTDIR"/..
New errors are not allowed. Warnings are strongly discouraged.
$ hg files "set:(**.py or **.txt) - tests/**" | sed 's|\\|/|g' |
> python contrib/check-config.py