cairo-perf-diff: Fix implementation of --help

Previously only "--" would givethe usage, and not "--help"
as intended. The new approach has been tested with ash, bash,
and dash, (hopefully that's good enough).

perf/cairo-perf-diff

@@ -49,7 +49,7 @@ END
 }
 
 # Yes, this ugly ad-hoc option parsing could be cleaned up
-if [ $# -eq 0 ] || [ "$1" = "--" ]; then
+if [ $# -eq 0 ] || [ "${1#-}" != "$1" ]; then
     usage
 fi