configitems: register the 'color.mode' config

mercurial/color.py

@@ -208,7 +208,7 b' def _modesetup(ui):'
     formatted = (always or (encoding.environ.get('TERM') != 'dumb'
                  and ui.formatted()))
 
-    mode = ui.config('color', 'mode', 'auto')
+    mode = ui.config('color', 'mode')
 
     # If pager is active, color.pagermode overrides color.mode.
     if getattr(ui, 'pageractive', False):

mercurial/configitems.py

@@ -56,6 +56,9 b' def getitemregister(configtable):'
 
 coreconfigitem = getitemregister(coreitems)
 
+coreconfigitem('color', 'mode',
+    default='auto',
+)
 coreconfigitem('devel', 'all-warnings',
     default=False,
 )