log-tree: allow to customize 'grafted' color

Commit 76f5df305b (log: decorate grafted commits with "grafted" -
2011-08-18) lets us decorate grafted commits but I forgot about the
color.decorate.* config.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/log-tree.c b/log-tree.c
index 30706fe..888c236 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -34,6 +34,7 @@ static const char *color_decorate_slots[] = {
 	[DECORATION_REF_TAG]	= "tag",
 	[DECORATION_REF_STASH]	= "stash",
 	[DECORATION_REF_HEAD]	= "HEAD",
+	[DECORATION_GRAFTED]	= "grafted",
 };
 
 static const char *decorate_get_color(int decorate_use_color, enum decoration_type ix)