tool: Support $NO_COLOR in ifchange

Author: nobu

tool/ifchange

@@ -20,6 +20,7 @@ timestamp=
 keepsuffix=
 srcavail=f
 color=auto
+[ "x${NO_COLOR-}" = x ] || color=never
 until [ $# -eq 0 ]; do
     case "$1" in
 	--)