repoview: improve compute staticblockers perf
Previously we would compute the repoview's static blockers by finding all the
children of hidden commits that were not hidden. This was O(number of commits
since first hidden change) since 'children' requires walking every commit from
tip until the first hidden change.
The new algorithm walks all heads down until it sees a public commit. This makes
the computation O(number of draft) commits, which is much faster in large
repositories with a large number of commits and a low number of drafts.
On a large repo with 1000+ obsolete markers and the earliest draft commit around
tip~200000, this improves computehidden perf by 200x (2s to 0.01s).