configitems: add a devel warning for extensions items overiding core one

configitems: add a devel warning for extensions items overiding core one

We do not want such case to pass silently. In the future we'll likely have
useful tool for an extension to alter the existing definition in core.

--- a/mercurial/configitems.py	Sat Jun 17 13:48:20 2017 +0200
+++ b/mercurial/configitems.py	Sun Jun 18 19:52:54 2017 +0200
@@ -16,7 +16,15 @@
 def loadconfigtable(ui, extname, configtable):
     """update config item known to the ui with the extension ones"""
     for section, items in configtable.items():
-        ui._knownconfig.setdefault(section, {}).update(items)
+        knownitems = ui._knownconfig.setdefault(section, {})
+        knownkeys = set(knownitems)
+        newkeys = set(items)
+        for key in sorted(knownkeys & newkeys):
+            msg = "extension '%s' overwrite config item '%s.%s'"
+            msg %= (extname, section, key)
+            ui.develwarn(msg, config='warn-config')
+
+        knownitems.update(items)
 
 class configitem(object):
     """represent a known config item

--- a/tests/test-devel-warnings.t	Sat Jun 17 13:48:20 2017 +0200
+++ b/tests/test-devel-warnings.t	Sun Jun 18 19:52:54 2017 +0200
@@ -203,14 +203,26 @@
   > cmdtable = {}
   > command = registrar.command(cmdtable)
   > 
+  > configtable = {}
+  > configitem = registrar.configitem(configtable)
+  > 
+  > configitem('test', 'some', default='foo')
+  > # overwrite a core config
+  > configitem('ui', 'quiet', default=False)
+  > configitem('ui', 'interactive', default=None)
+  > 
   > @command(b'buggyconfig')
   > def cmdbuggyconfig(ui, repo):
   >     repo.ui.config('ui', 'quiet', False)
   >     repo.ui.config('ui', 'interactive', None)
+  >     repo.ui.config('test', 'some', 'foo')
   > EOF
 
   $ hg --config "extensions.buggyconfig=${TESTTMP}/buggyconfig.py" buggyconfig
+  devel-warn: extension 'buggyconfig' overwrite config item 'ui.interactive' at: */mercurial/extensions.py:* (loadall) (glob)
+  devel-warn: extension 'buggyconfig' overwrite config item 'ui.quiet' at: */mercurial/extensions.py:* (loadall) (glob)
   devel-warn: specifying a default value for a registered config item: 'ui.quiet' 'False' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
   devel-warn: specifying a default value for a registered config item: 'ui.interactive' 'None' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
+  devel-warn: specifying a default value for a registered config item: 'test.some' 'foo' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
 
   $ cd ..