hgweb: filter graphmod.colored() output before iterating over it Consumers in this function use output of graphmod.colored(), but only want items with type == CHANGESET, so let's filter it early. This is primarily just a refactoring, but it also fixes a potential small bug with `rows = len(tree)` (this variable is used for "Rows shown" line in raw-graph) if there are items of other types.