color: concatenate effects correctly and avoid printing empty effects

hgext/color.py

@@ -134,9 +134,12 b' def configstyles(ui):'
 
 _buffers = None
 def style(msg, label):
-    effects = ''
+    effects = []
     for l in label.split():
-        effects += _styles.get(l, '')
+        s = _styles.get(l, '')
+        if s:
+            effects.append(s)
+    effects = ''.join(effects)
     if effects:
         return '\n'.join([render_effects(s, effects)
                           for s in msg.split('\n')])