clfilter: use filtering in `visibleheads`

This is the second real use of changelog filtering. The change is very small to
allow testing the new filter with a setup close to the original one.

We replace custom post processing on `heads`function by call to the standard
code pass on a filtering repo.

In later coming will have wider usage of filtering that will make the dedicated
function useless.

mercurial/discovery.py

@@ -338,19 +338,7 b' def checkheads(repo, remote, outgoing, r'
 
 def visibleheads(repo):
     """return the set of visible head of this repo"""
-    # XXX we want a cache on this
-    sroots = repo._phasecache.phaseroots[phases.secret]
-    if sroots or repo.obsstore:
-        # XXX very slow revset. storing heads or secret "boundary"
-        # would help.
-        revset = repo.set('heads(not (%ln:: + extinct()))', sroots)
-
-        vheads = [ctx.node() for ctx in revset]
-        if not vheads:
-            vheads.append(nullid)
-    else:
-        vheads = repo.heads()
-    return vheads
+    return repo.filtered('unserved').heads()
 
 
 def visiblebranchmap(repo):

tests/test-obsolete-divergent.t

@@ -91,7 +91,7 b' check that mercurial refuse to push'
   $ hg push ../other
   pushing to ../other
   searching for changes
-  abort: push includes divergent changeset: 82623d38b9ba!
+  abort: push includes divergent changeset: 392fd25390da!
   [255]
 
   $ cd ..

tests/test-obsolete.t

@@ -176,6 +176,7 b" And that we can't push bumped changeset"
   $ hg push ../tmpa -r 0 --force #(make repo related)
   pushing to ../tmpa
   searching for changes
+  warning: repository is unrelated
   adding changesets
   adding manifests
   adding file changes