add a user message during --show-prompts

aider/coders/base_coder.py

@@ -418,7 +418,7 @@ class Coder:
         if repo_content:
             if all_content:
                 all_content += "\n"
-            all_content += repo_content
+            all_content += repo_content + "\n"
 
         if self.abs_fnames:
             files_content = self.gpt_prompts.files_content_prefix

aider/main.py

@@ -342,6 +342,9 @@ def main(argv=None, input=None, output=None, force_git_root=None, return_coder=F
     coder.show_announcements()
 
     if args.show_prompts:
+        coder.cur_messages += [
+            dict(role="user", content="Hello!"),
+        ]
         messages = coder.format_messages()
         utils.show_messages(messages)
         return