config: remove pycompat.bytestr() for defaultvalue

This is a follow-up patch to 51a2e3102db2. This removes
`pycompat.bytestr` to preserve `None` in `commands.config()`.

Differential Revision: https://phab.mercurial-scm.org/D6712

mercurial/commands.py

@@ -1872,7 +1872,7 b' def config(ui, repo, *values, **opts):'
     for section, name, value in ui.walkconfig(untrusted=untrusted):
         source = ui.configsource(section, name, untrusted)
         value = pycompat.bytestr(value)
-        defaultvalue = pycompat.bytestr(ui.configdefault(section, name))
+        defaultvalue = ui.configdefault(section, name)
         if fm.isplain():
             source = source or 'none'
             value = value.replace('\n', '\\n')

tests/test-config.t

@@ -70,7 +70,7 b' Test case sensitive configuration'
   $ hg showconfig Section.KeY -Tjson
   [
    {
-    "defaultvalue": "None",
+    "defaultvalue": null,
     "name": "Section.KeY",
     "source": "*.hgrc:*", (glob)
     "value": "Case Sensitive"
@@ -103,7 +103,7 b' Test empty config source:'
   $ hg config empty.source -Tjson
   [
    {
-    "defaultvalue": "None",
+    "defaultvalue": null,
     "name": "empty.source",
     "source": "",
     "value": "value"