tracediff: Rename --diff=TOOL option to --tool=TOOL.
tracediff: Rename --diff=TOOL option to --tool=TOOL.

More intuitive (I was always mistyping it) as it matches valgrind.

trtt/apitrace,trtt/apitrace,trtt/apitrace,joshua5201/apitrace,schulmar/apitrace,surround-io/apitrace,schulmar/apitrace,apitrace/apitrace,tuanthng/apitrace,swq0553/apitrace,schulmar/apitrace,tuanthng/apitrace,swq0553/apitrace,EoD/apitrace,joshua5201/apitrace,swq0553/apitrace,swq0553/apitrace,surround-io/apitrace,apitrace/apitrace,EoD/apitrace,trtt/apitrace,joshua5201/apitrace,tuanthng/apitrace,surround-io/apitrace,EoD/apitrace,joshua5201/apitrace,apitrace/apitrace,swq0553/apitrace,surround-io/apitrace,trtt/apitrace,joshua5201/apitrace,EoD/apitrace,schulmar/apitrace,tuanthng/apitrace,apitrace/apitrace,EoD/apitrace,surround-io/apitrace,schulmar/apitrace,tuanthng/apitrace