performance: speedup computation of mutable revisions

In their current state, revset calls can be very costly, as we test
predicates on the entire repository. The "mutable" filter is used
during branch cache loading operation. 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,
- 1646 mutable revision

Before:
  ! mutable
  ! wall 0.032405

After:
  ! mutable
  ! wall 0.001469

Performance test on a Mozilla central checkout:
- 117293 total changesets,
- 1 mutable changeset,

Before:
  ! mutable
  ! wall 0.188636

After:
  ! mutable
  ! wall 0.000022

mercurial/repoview.py

@@ -58,7 +58,9 b' def computemutable(repo):'
     assert not repo.changelog.filteredrevs
     # fast check to avoid revset call on huge repo
     if util.any(repo._phasecache.phaseroots[1:]):
-        return frozenset(repo.revs('draft() + secret()'))
+        getphase = repo._phasecache.phase
+        maymutable = filteredrevs(repo, 'impactable')
+        return frozenset(r for r in maymutable if getphase(repo, r))
     return frozenset()
 
 def computeimpactable(repo):