feat: conditionally execute the post-stack-set call

If the value does not change, then we should not trigger the command

docs/deployment/methods/herokuish-buildpacks.md

@@ -140,6 +140,8 @@ The specified stack can also be unset by omitting the name of the stack when cal
 dokku buildpacks:stack-set node-js-app
 ```
 
+A change in the stack value will execute the `post-stack-set` trigger.
+
 Finally, stacks can be set or unset globally as a fallback. This will take precedence over a globally set `DOKKU_IMAGE` environment variable (`gliderlabs/herokuish:latest` by default).
 
 ```shell

plugins/buildpacks/subcommands.go

@@ -141,6 +141,11 @@ func CommandSet(appName string, buildpack string, index int) error {
 
 // CommandStackSet implements buildpacks:stack-set
 func CommandStackSet(appName string, stack string) error {
+	oldStack := common.PropertyGet("buildpacks", appName, "stack")
 	common.CommandPropertySet("buildpacks", appName, "stack", stack, DefaultProperties, GlobalProperties)
-	return common.PlugnTrigger("post-stack-set", []string{appName, stack}...)
+	if oldStack != stack {
+		return common.PlugnTrigger("post-stack-set", []string{appName, stack}...)
+	}
+
+	return nil
 }