mercurial/templates/json/changelist.tmpl

tests: avoid implicit conversion of str to unicode

On Python 2, str.encode('utf-8') implicitly converts the string to unicode and
then back to str.

Using _sys2bytes() ensures that opt is only encoded on Python 3, where opt is
unicode.

Although contrived, before this change, a UnicodeDecodeError could be triggered
on Python 2 when passing non-ascii values to --extra-config-opt.

\{
  "node": {node|json},
  "changeset_count": {changesets|json},
  "changesets": [{join(entries%changelistentry, ", ")}]
}