color: extract color and effect display from 'debugcolor'

We are about to introduce a second mode for 'hg debugcolor' that would list the
known label and their configuration, so we split the code related to color and
effect out of the main function.

hgext/color.py

@@ -539,6 +539,10 b' def extsetup(ui):'
 @command('debugcolor', [], 'hg debugcolor')
 def debugcolor(ui, repo, **opts):
     """show available colors and effects"""
+    ui.write(('color mode: %s\n') % ui._colormode)
+    return _debugdisplaycolor(ui)
+
+def _debugdisplaycolor(ui):
     global _styles
     oldstyle = _styles
     try:
@@ -551,7 +555,6 b' def debugcolor(ui, repo, **opts):'
                     _styles[k] = k[6:]
                 elif k.startswith('terminfo.'):
                     _styles[k] = k[9:]
-        ui.write(('color mode: %s\n') % ui._colormode)
         ui.write(_('available colors:\n'))
         for colorname, label in _styles.items():
             ui.write(('%s\n') % colorname, label=label)