don't let -debug affect the build cache hashes

Noticed while debugging #492,
as adding -debug to a previously run command would unexpectedly
rebuild all packages in the build, as if -a was given.

While here, remove commented out testscript that were kept in error.

hash.go

@@ -118,7 +118,11 @@ func appendFlags(w io.Writer, forBuildHash bool) {
 	if flagTiny {
 		io.WriteString(w, " -tiny")
 	}
-	if flagDebug {
+	if flagDebug && !forBuildHash {
+		// -debug doesn't affect the build result at all,
+		// so don't give it separate entries in the build cache.
+		// If the user really wants to see debug info for already built deps,
+		// they can use "go clean cache" or the "-a" build flag to rebuild.
 		io.WriteString(w, " -debug")
 	}
 	if flagDebugDir != "" && !forBuildHash {

testdata/scripts/debugdir.txt

@@ -1,8 +1,5 @@
 env GOGARBLE=*
 
-# ! garble -debug -debugdir ./debug1 build
-# cp stderr /tmp/log
-# exit
 garble -debugdir ./debug1 build
 exists 'debug1/test/main/imported/imported.go' 'debug1/test/main/main.go' 'debug1/reflect/type.go'
 ! grep ImportedFunc $WORK/debug1/test/main/imported/imported.go