performance: speedup computation of unserved revisions

In their current state, revset calls can be very costly, as we test
predicates on the entire repository. The "unserved" filter is used
in multiple applications, and in particular in some branch cache
loading operations. We need to make it fast.

This change drops revset calls in favor of direct testing of the
phase of a changeset.

Performance test on my Mercurial checkout

- 19857 total changesets,
- 1584 obsolete changesets,
- 13310 obsolescence markers.

Before:
  ! unserved
  ! wall 0.030477

After:
  ! unserved
  ! wall 0.011844

Performance test on a Mozilla central checkout:

- 117293 total changesets,
- 1 obsolete changeset,
- 1 obsolescence marker.

Before:
  ! unserved
  ! wall 0.111259

After:
  ! unserved
  ! wall 0.000084

mercurial/repoview.py

@@ -38,8 +38,17 b' def computeunserved(repo):'
     Secret and hidden changeset should not pretend to be here."""
     assert not repo.changelog.filteredrevs
     # fast path in simple case to avoid impact of non optimised code
-    if phases.hassecret(repo) or repo.obsstore:
-        return frozenset(repo.revs('hidden() + secret()'))
+    hiddens = filteredrevs(repo, 'hidden')
+    if phases.hassecret(repo):
+        cl = repo.changelog
+        secret = phases.secret
+        getphase = repo._phasecache.phase
+        first = min(cl.rev(n) for n in repo._phasecache.phaseroots[secret])
+        revs = cl.revs(start=first)
+        secrets = set(r for r in revs if getphase(repo, r) >= secret)
+        return frozenset(hiddens | secrets)
+    else:
+        return hiddens
     return frozenset()
 
 def computemutable(repo):