py3: don't risk passing a None value to error.ManifestLookupError()

This makes the test case added in 20dd2a259b0f (test-grep: add tests
for --follow with/without --diff and/or paths, 2020-09-11) pass on
Python 3.

Differential Revision: https://phab.mercurial-scm.org/D9030

mercurial/context.py

@@ -271,7 +271,7 b' class basectx(object):'
                 return self._manifest.find(path)
             except KeyError:
                 raise error.ManifestLookupError(
-                    self._node, path, _(b'not found in manifest')
+                    self._node or b'None', path, _(b'not found in manifest')
                 )
         if '_manifestdelta' in self.__dict__ or path in self.files():
             if path in self._manifestdelta:
@@ -284,7 +284,7 b' class basectx(object):'
             node, flag = mfl[self._changeset.manifest].find(path)
         except KeyError:
             raise error.ManifestLookupError(
-                self._node, path, _(b'not found in manifest')
+                self._node or b'None', path, _(b'not found in manifest')
             )
 
         return node, flag