color: add global option in extsetup() instead of globally

This prevents side effects from occurring when importing the color
extension.

hgext/color.py

@@ -223,6 +223,7 b' def uisetup(ui):'
         return orig(ui_, opts, cmd, cmdfunc)
     extensions.wrapfunction(dispatch, '_runcommand', colorcmd)
 
+def extsetup(ui):
 commands.globalopts.append(
     ('', 'color', 'auto',
      _("when to colorize (boolean, always, auto, or never)"),