cli/command: PromptUserForCredentials: move trimming where it's used

- move trimming defaultUsername inside the if-branch, as it's the only
  location where the result of the trimmed username is use.
- do the reverse for trimming argUser, because the result of trimming
  argUser is used outside of the if-branch (not just for the condition).
  putting it inside the condition makes it easy to assume the result is
  only used locally.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit eda78e9cdc)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/registry.go

@@ -125,9 +125,8 @@ func PromptUserForCredentials(ctx context.Context, cli Cli, argUser, argPassword
 		cli.SetIn(streams.NewIn(os.Stdin))
 	}
 
-	defaultUsername = strings.TrimSpace(defaultUsername)
-
-	if argUser = strings.TrimSpace(argUser); argUser == "" {
+	argUser = strings.TrimSpace(argUser)
+	if argUser == "" {
 		if serverAddress == registry.IndexServer {
 			// if this is a default registry (docker hub), then display the following message.
 			fmt.Fprintln(cli.Out(), "Log in with your Docker ID or email address to push and pull images from Docker Hub. If you don't have a Docker ID, head over to https://hub.docker.com/ to create one.")
@@ -138,6 +137,7 @@ func PromptUserForCredentials(ctx context.Context, cli Cli, argUser, argPassword
 		}
 
 		var prompt string
+		defaultUsername = strings.TrimSpace(defaultUsername)
 		if defaultUsername == "" {
 			prompt = "Username: "
 		} else {