localrepo: micro-optimize __len__() to bypass repoview

Since len(changelog) isn't overridden, we don't have to validate a cache of
unfiltered changelog.

  $ python -m timeit -n 10000 \
           -s 'from mercurial import hg, ui; repo = hg.repository(ui.ui());' \
           'len(repo)'
  orig) 10000 loops, best of 3: 32.1 usec per loop
  new)  10000 loops, best of 3: 1.79 usec per loop

Spotted by Jordi Gutiérrez Hermoso.

mercurial/localrepo.py

@@ -762,7 +762,9 class localrepository(object):
     __bool__ = __nonzero__
 
     def __len__(self):
-        return len(self.changelog)
+        # no need to pay the cost of repoview.changelog
+        unfi = self.unfiltered()
+        return len(unfi.changelog)
 
     def __iter__(self):
         return iter(self.changelog)