color: call correct superclass method in write_err

Without this fix, any calls to write_err would go to stdout instead of
stderr, and calls during pushbuffer would cause unpack ValueErrors on
popbuffer.

tests/test-ui-color.py

@@ -0,0 +1,11 b''
+#!/usr/bin/env python
+
+from hgext import color
+
+# ensure errors aren't buffered
+testui = color.colorui()
+testui.pushbuffer()
+testui.write('buffered\n')
+testui.warn('warning\n')
+testui.write_err('error\n')
+print repr(testui.popbuffer())

tests/test-ui-color.py.out

@@ -0,0 +1,3 b''
+warning
+error
+'buffered\n'

hgext/color.py

@@ -165,7 +165,7 b' class colorui(uimod.ui):'
             for a in args:
                 win32print(a, super(colorui, self).write_err, **opts)
         else:
-            return super(colorui, self).write(
+            return super(colorui, self).write_err(
                 *[self.label(str(a), label) for a in args], **opts)
 
     def label(self, msg, label):