configitems: traverse sections deterministically stable

configitems: traverse sections deterministically

Otherwise output can be non-deterministic if there are warnings
for multiple sections.

Differential Revision: https://phab.mercurial-scm.org/D1947

--- a/mercurial/configitems.py	Mon Jan 29 21:42:18 2018 -0500
+++ b/mercurial/configitems.py	Mon Jan 29 13:30:29 2018 -0800
@@ -17,7 +17,7 @@
 
 def loadconfigtable(ui, extname, configtable):
     """update config item known to the ui with the extension ones"""
-    for section, items in configtable.items():
+    for section, items in sorted(configtable.items()):
         knownitems = ui._knownconfig.setdefault(section, itemregister())
         knownkeys = set(knownitems)
         newkeys = set(items)