aider: Handle the case where the repository has never had any commits by using index.diff(None) to get the staged files.

# Aider chat conversation:

USER: index.diff("HEAD") fails when the repo has never had any commits

ASSISTANT: To handle the case where the repository has never had any commits, we can add a try-except block around the `index.diff("HEAD")` call in the `get_tracked_files` method of the `GitRepo` class in the `aider/repo.py` file. If a `git.exc.BadName` exception is raised, we can assume that there are no commits and return an empty list for the staged files.

Here is the proposed change:

```python
aider/repo.py
<<<<<<< ORIGINAL
        # Add staged files
        index = self.repo.index
        staged_files = [item.a_path for item in index.diff("HEAD")]
        files.extend(staged_files)
=======
        # Add staged files

^C KeyboardInterrupt

USER: there can be staged files when there are not commits

ASSISTANT: You're correct. If there are no commits, `index.diff("HEAD")` will fail, but there can still be staged files. We can use `index.diff(None)` to get the staged files in this case. Here's the proposed change:

```python
aider/repo.py
<<<<<<< ORIGINAL
        # Add staged files
        index = self.repo.index
        staged_files = [item.a_path for item in index.diff("HEAD")]
        files.extend(staged_files)
=======
        # Add staged files
        index = self.repo.index
        try:
            staged_files = [item.a_path for item in index.diff("HEAD")]
        except git.exc.BadName:
            staged_files = [item.a_path for item in index.diff(None)]
        files.extend(staged_files)
>>>>>>> UPDATED
```

aider/repo.py

@@ -156,7 +156,10 @@ class GitRepo:
 
         # Add staged files
         index = self.repo.index
-        staged_files = [item.a_path for item in index.diff("HEAD")]
+        try:
+            staged_files = [item.a_path for item in index.diff("HEAD")]
+        except git.exc.BadName:
+            staged_files = [item.a_path for item in index.diff(None)]
         files.extend(staged_files)
 
         # convert to appropriate os.sep, since git always normalizes to /