graphmod: attempt to clarify documentation of groupbranchiter() Thanks to Pierre-Yves for checking my cleanups here and helping me understand the algorithm well enough to help document it.

graphmod: rename graph-topological config to graph-group-branches The latter suggests what the change is slightly better.

groupbranchiter: allow callers to select the first branch Instead of just bootstrapping the algorithm with the first revision we see, allow callers to pass revs that should be displayed first. All branches are retained until we can display such revision. Expected usage is to display the current working copy parent first.