manifest: also declare treemanifest as implementing imanifestdict

It always has been intended to be, but it had a mismatched flags()
method until the previous change.

Differential Revision: https://phab.mercurial-scm.org/D8366

mercurial/manifest.py

@@ -760,6 +760,7 b' def _splittopdir(f):'
 _noop = lambda s: None
 
 
[e-mail](repository.imanifestdict)
 class treemanifest(object):
     def __init__(self, dir=b'', text=b''):
         self._dir = dir

tests/test-check-interfaces.py

@@ -211,6 +211,7 b' def main():'
 
     ziverify.verifyClass(repository.ifilestorage, filelog.filelog)
     ziverify.verifyClass(repository.imanifestdict, manifest.manifestdict)
+    ziverify.verifyClass(repository.imanifestdict, manifest.treemanifest)
     ziverify.verifyClass(
         repository.imanifestrevisionstored, manifest.manifestctx
     )