color: handle 'ui.plain()' directly in mode setup

If 'ui.plain()' is set we should not colorize. We move that logic into the
function that determine and setup the color mode. As all other code respect
the resulting mode this will be equivalent.

hgext/color.py

@@ -189,8 +189,6 b' command = cmdutil.command(cmdtable)'
 testedwith = 'ships-with-hg-core'
 
 def uisetup(ui):
-    if ui.plain():
-        return
     def colorcmd(orig, ui_, opts, cmd, cmdfunc):
         mode = color._modesetup(ui_, opts['color'])
         uimod.ui._colormode = mode

mercurial/color.py

@@ -162,6 +162,8 b' def _terminfosetup(ui, mode):'
         _terminfo_params.clear()
 
 def _modesetup(ui, coloropt):
+    if ui.plain():
+        return None
     if coloropt == 'debug':
         return 'debug'