Detect buildpacks for specific branch (fall back to deploy-branch to be consistent with deployment behavior)

plugins/buildpacks/src/commands/commands.go

@@ -20,7 +20,7 @@ Additional commands:`
 	helpContent = `
     buildpacks:add [--index 1] <app> <buildpack>, Add new app buildpack while inserting into list of buildpacks if necessary
     buildpacks:clear <app>, Clear all buildpacks set on the app
-    buildpacks:detect <app>, Detect the buildpack for an app
+    buildpacks:detect <app> [--branch <branch-name>], Detect the buildpack for an app (if not specified, the branch flag defaults to the deploy branch)
     buildpacks:list <app>, List all buildpacks for an app
     buildpacks:remove <app> <buildpack>, Remove a buildpack set on the app
     buildpacks:report [<app>] [<flag>], Displays a buildpack report for one or more apps

plugins/buildpacks/src/subcommands/subcommands.go

@@ -32,9 +32,13 @@ func main() {
 		err = buildpacks.CommandClear(appName)
 	case "detect":
 		args := flag.NewFlagSet("buildpacks:detect", flag.ExitOnError)
+		branch := args.String("branch", "", "--branch: the branch to detect against (defaults to the deploy branch)")
 		args.Parse(os.Args[2:])
 		appName := args.Arg(0)
-		err = buildpacks.CommandDetect(appName)
+		if *branch == "" {
+            *branch = common.PropertyGet("git", appName, "deploy-branch")
+		}
+		err = buildpacks.CommandDetect(appName, *branch)
 	case "list":
 		args := flag.NewFlagSet("buildpacks:list", flag.ExitOnError)
 		args.Parse(os.Args[2:])