annotate: prepare ancestry context of workingfilectx

_ancestrycontext is necessary for fast lookup of _changeid. Because we can't
compute the ancestors from wctx, we skip to its parents. 'None' is not needed
to be included in _ancestrycontext because it is used for a membership test
of filelog revisions.

repo:    https://hg.mozilla.org/releases/mozilla-beta/#062e49bcb2da
command: hg annotate -r 'wdir()' gfx/thebes/gfxWindowsPlatform.cpp
before:  51.520 sec
after:    1.780 sec

mercurial/context.py

@@ -906,8 +906,15 b' class basefilectx(object):'
         introrev = self.introrev()
         if self.rev() != introrev:
             base = self.filectx(self.filenode(), changeid=introrev)
-        if introrev and getattr(base, '_ancestrycontext', None) is None:
-            ac = self._repo.changelog.ancestors([introrev], inclusive=True)
+        if getattr(base, '_ancestrycontext', None) is None:
+            cl = self._repo.changelog
+            if introrev is None:
+                # wctx is not inclusive, but works because _ancestrycontext
+                # is used to test filelog revisions
+                ac = cl.ancestors([p.rev() for p in base.parents()],
+                                  inclusive=True)
+            else:
+                ac = cl.ancestors([introrev], inclusive=True)
             base._ancestrycontext = ac
 
         # This algorithm would prefer to be recursive, but Python is a