discovery: log information about obshashrange We augment the blackbox output to help with data gathering.

obshistory: add a graph option on the debugobshistory command Add a graph option (--graph) to the debugobshistory. The output is like the 'hg log -G' output. The option is activated by default but can be deactivated with '--no-graph' option. There are various issue with the current implementation (multiple cycles handling, N² complexity) but this can be fixed later.

obshistory: import 'node' as 'nodemod' This simplify the next changeset.