phases: check secret presence the right way during discovery

There is an official function for this, lets use it.

This will prevent the code to break in the future while we refactor the phase
code.

mercurial/discovery.py

@@ -168,7 +168,7 b' def findcommonoutgoing('
     og.commonheads, _any, _hds = commoninc
 
     # compute outgoing
-    mayexclude = repo._phasecache.phaseroots[phases.secret] or repo.obsstore
+    mayexclude = phases.hassecret(repo) or repo.obsstore
     if not mayexclude:
         og.ancestorsof = onlyheads or repo.heads()
     elif onlyheads is None: