[PATCH 01 of 10] config: explicitly track the use of the standard default value

[Pierre-Yves David]

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at octobus.net>
# Date 1497696671 -7200
#      Sat Jun 17 12:51:11 2017 +0200
# Node ID 5e988d4f834b981d8aaf9d77019550d3800687d5
# Parent  29558247b00eff8c95c7604032b59cfbab34010d
# EXP-Topic config.register
# Available At https://www.mercurial-scm.org/repo/users/marmoute/mercurial/
#              hg pull https://www.mercurial-scm.org/repo/users/marmoute/mercurial/ -r 5e988d4f834b
config: explicitly track the use of the standard default value

We introduce a small object used to detect that no specific default value has
been passed to 'ui.config'. We need this explicit special value since "None" is
a valid and common default value.

The end goal here is to make progress on a centralised and explicit declaration
of the available config option. A first good usecase for this are "default"
value.  Before starting looking further down this alley we needs to rework the
handling of default value in the 'ui' object to have all configxyz methods going
through the same logic. This is the first changeset on this trek.

diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -140,6 +140,10 @@ class httppasswordmgrdbproxy(object):
 def _catchterm(*args):
     raise error.SignalInterrupt
 
+# unique object used to detect no default value has been provided when
+# retrieving configuration value.
+_unset = object()
+
 class ui(object):
     def __init__(self, src=None):
         """Create a fresh new ui object if no src given
@@ -394,7 +398,9 @@ class ui(object):
     def configsource(self, section, name, untrusted=False):
         return self._data(untrusted).source(section, name)
 
-    def config(self, section, name, default=None, untrusted=False):
+    def config(self, section, name, default=_unset, untrusted=False):
+        if default is _unset:
+            default = None
         if isinstance(name, list):
             alternates = name
         else: