context: call normal on the right object

dirstate.normal is the method that marks files as unchanged/normal.

Rev 20a30cd41d21 started caching dirstate.normal in order to improve
performance. However, there was an error in the patch: taking the wlock, under
some conditions depending on platform, can cause a new dirstate object to be
created. Caching dirstate.normal before calling wlock would then cause the
fixup calls below to be on the old dirstate object, effectively disappearing
into the ether.

On Unix and Unix-like OSes, the condition under which we create a new dirstate
object is 'the dirstate file has been modified since the last time we opened
it'. This happens pretty rarely, so the object is usually the same -- there's
little impact.

On Windows, the condition is 'always'. This means files in the lookup state are
never marked normal, so the bug has a serious performance impact since all the
files in the lookup state are re-read every time hg status is run.

mercurial/context.py

@@ -1341,8 +1341,10 b' class workingctx(committablectx):'
             try:
                 # updating the dirstate is optional
                 # so we don't wait on the lock
+                # wlock can invalidate the dirstate, so cache normal _after_
+                # taking the lock
+                wlock = self._repo.wlock(False)
                 normal = self._repo.dirstate.normal
-                wlock = self._repo.wlock(False)
                 try:
                     for f in fixup:
                         normal(f)