obscheckhead: skip context creation to test phase

On repository with many many heads, this is called often enough to show an
overhead to appears in some profile. So we skip the 'changectx' creation to test
the node phases.

mercurial/discovery.py

@@ -431,6 +431,7 b' def _postprocessobsolete(pushop, futurec'
     repo = pushop.repo
     unfi = repo.unfiltered()
     tonode = unfi.changelog.node
+    torev = unfi.changelog.rev
     public = phases.public
     getphase = unfi._phasecache.phase
     ispublic = (lambda r: getphase(unfi, r) == public)
@@ -460,8 +461,7 b' def _postprocessobsolete(pushop, futurec'
     while localcandidate:
         nh = localcandidate.pop()
         # run this check early to skip the evaluation of the whole branch
-        if (nh in futurecommon
-                or unfi[nh].phase() <= public):
+        if (nh in futurecommon or ispublic(torev(nh))):
             newhs.add(nh)
             continue