treemanifest: add an optimized __nonzero__()

We use bool(manifest) in at least some places:

  localrepo.py:1730
  hgweb/webcommands.py:524

Since the treemanifest class doesn't define __nonzero__() (before this
patch), bool(manifest) will instead call __len__(), which can be slow
for treemanifests. This patch may make a noticeable difference in the
localrepo case above, but that only happens when committing a merge
and I haven't timed it.

Note that Durham already added a __nonzero__ implementation to
manifestdict in b19291e5d506 (manifest: add __nonzero__ method,
2016-11-03).

Differential Revision: https://phab.mercurial-scm.org/D2232

mercurial/manifest.py

@@ -755,6 +755,12 b' class treemanifest(object):'
             size += m.__len__()
         return size
 
+    def __nonzero__(self):
+        # Faster than "__len() != 0" since it avoids loading sub-manifests
+        return not self._isempty()
+
+    __bool__ = __nonzero__
+
     def _isempty(self):
         self._load() # for consistency; already loaded by all callers
         return (not self._files and (not self._dirs or