tests: add bytes prefixes to test-histedit-fold.t's local extension

tests: add bytes prefixes to test-histedit-fold.t's local extension

# skip-blame b prefixes

Differential Revision: https://phab.mercurial-scm.org/D2246

--- a/tests/test-histedit-fold.t	Tue Feb 13 18:25:05 2018 -0500
+++ b/tests/test-histedit-fold.t	Tue Feb 13 18:47:55 2018 -0500
@@ -154,9 +154,9 @@
   > from mercurial import util
   > def abortfolding(ui, repo, hooktype, **kwargs):
   >     ctx = repo[kwargs.get('node')]
-  >     if set(ctx.files()) == {'c', 'd', 'f'}:
+  >     if set(ctx.files()) == {b'c', b'd', b'f'}:
   >         return True # abort folding commit only
-  >     ui.warn('allow non-folding commit\\n')
+  >     ui.warn(b'allow non-folding commit\\n')
   > EOF
   $ cat > .hg/hgrc <<EOF
   > [hooks]