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