hidden: remove unnecessary guard condition

The "if visible" guard is now pretty pointless, because the function
call it guards will do almost no work anyway when there are no visible
revisions.

We can also stop wrapping "visible" in a set since it just needs to be
an iterable now.

mercurial/repoview.py

@@ -76,9 +76,8 b' def computehidden(repo):'
         mutablephases = (phases.draft, phases.secret)
         mutable = repo._phasecache.getrevset(repo, mutablephases)
 
-        visible = set(mutable - hidden)
-        if visible:
-            _revealancestors(pfunc, hidden, visible)
+        visible = mutable - hidden
+        _revealancestors(pfunc, hidden, visible)
     return frozenset(hidden)
 
 def computeunserved(repo):