phases: add a getrevset method to phasecache

This is part of a refactoring that moves some phase query optimization from
revset.py to phases.py.

The motivation behind this was chg repo preloading - to make the obsstore
depend on less things (like the revset language). The refactoring also looks
good by itself - phasecache does not expose its private field "_phasesets"
via public methods and revset.py is accessing it in a hacky way.

This patch adds a "getrevset" method, which takes multiple phases and
returns a revset in an best-effort efficient way - for "public" phase, it
returns a lazy generatorset; for "draft" and "secret", it returns efficient
"baseset".

mercurial/phases.py

@@ -115,6 +115,7 b' from .node import ('
 from . import (
     encoding,
     error,
+    smartset,
 )
 
 allphases = public, draft, secret = range(3)
@@ -170,6 +171,27 b' class phasecache(object):'
             self.filterunknown(repo)
             self.opener = repo.svfs
 
+    def getrevset(self, repo, phases):
+        """return a smartset for the given phases"""
+        self.loadphaserevs(repo) # ensure phase's sets are loaded
+
+        if self._phasesets and all(self._phasesets[p] is not None
+                                   for p in phases):
+            # fast path - use _phasesets
+            revs = self._phasesets[phases[0]]
+            if len(phases) > 1:
+                revs = revs.copy() # only copy when needed
+                for p in phases[1:]:
+                    revs.update(self._phasesets[p])
+            if repo.changelog.filteredrevs:
+                revs = revs - repo.changelog.filteredrevs
+            return smartset.baseset(revs)
+        else:
+            # slow path - enumerate all revisions
+            phase = self.phase
+            revs = (r for r in repo if phase(repo, r) in phases)
+            return smartset.generatorset(revs, iterasc=True)
+
     def copy(self):
         # Shallow copy meant to ensure isolation in
         # advance/retractboundary(), nothing more.