dagop: change revdescendants() to include all root revisions
Prepares for adding depth support. I want to process depth=0 in
revdescendants() to make things simpler.
only() also calls dagop.revdescendants(), but it filters out root revisions
explicitly. So this should cause no problem.
# descendants(0) using hg repo
0) 0.052380
1) 0.051226
# only(tip) using hg repo
0) 0.001433
1) 0.001425