Prevent setting a public upstream when creating a private repo.

Fixes #1854

commands/create.go

@@ -110,7 +110,18 @@ func create(command *Command, args *Args) {
 	gh := github.NewClient(project.Host)
 
 	if gh.IsRepositoryExist(project) {
-		ui.Errorln("Existing repository detected. Updating git remote")
+
+		repo, err := gh.Repository(project)
+		if err != nil {
+			utils.Check(err)
+		}
+
+		if !repo.Private && flagCreatePrivate {
+			err = fmt.Errorf("Existing public repository detected. git remote not updated")
+			utils.Check(err)
+		} else {
+			ui.Errorln("Existing repository detected. Updating git remote")
+		}
 	} else {
 		if !args.Noop {
 			repo, err := gh.CreateRepository(project, flagCreateDescription, flagCreateHomepage, flagCreatePrivate)