context: use unfiltered repo for '.'

There is no reason to read obsolescence markers when doing a plain 'hg
status' without --rev. Use the unfiltered repo when initializing
context._rev to speed things up. This speeds up 'hg status' from
1.342s to 0.080s on my repo with ~110k markers.

mercurial/context.py

@@ -376,10 +376,6 b' class changectx(basectx):'
                 return
             if isinstance(changeid, long):
                 changeid = str(changeid)
-            if changeid == '.':
-                self._node = repo.dirstate.p1()
-                self._rev = repo.changelog.rev(self._node)
-                return
             if changeid == 'null':
                 self._node = nullid
                 self._rev = nullrev
@@ -388,6 +384,12 b' class changectx(basectx):'
                 self._node = repo.changelog.tip()
                 self._rev = repo.changelog.rev(self._node)
                 return
+            if changeid == '.' or changeid == repo.dirstate.p1():
+                # this is a hack to delay/avoid loading obsmarkers
+                # when we know that '.' won't be hidden
+                self._node = repo.dirstate.p1()
+                self._rev = repo.unfiltered().changelog.rev(self._node)
+                return
             if len(changeid) == 20:
                 try:
                     self._node = changeid