annotate: always prepare ancestry context of base fctx (issue4600) stable

annotate: always prepare ancestry context of base fctx (issue4600)

This patch extends the workaround introduced by dd01834a696f. Even if the
base fctx is the same as intorrev, _ancestrycontext must be built for faster
_changeid lookup.

repo:    https://hg.mozilla.org/releases/mozilla-beta
command: hg annotate -r 4954faa47dd0 gfx/thebes/gfxWindowsPlatform.cpp
before:  52.450 sec
after:    1.820 sec

--- a/mercurial/context.py	Sat Apr 04 21:54:12 2015 -0700
+++ b/mercurial/context.py	Thu Apr 16 22:33:53 2015 +0900
@@ -919,6 +919,7 @@
         introrev = self.introrev()
         if self.rev() != introrev:
             base = self.filectx(self.filenode(), changeid=introrev)
+        if getattr(base, '_ancestrycontext', None) is None:
             ac = self._repo.changelog.ancestors([introrev], inclusive=True)
             base._ancestrycontext = ac