git_ui: Add collapse_untracked_diff settings to improve usability for untracked files (#32591)

In repositories with untracked files that are not intended to be added,
showing the expanded diffs in the panel is confusing, as it places the
changes side by side with changes in tracked files.

This change adds a setting, collapse_untracked_diff, that can be enabled
to collapse untracked diffs by default when the panel is opened.

See
https://github.com/zed-industries/zed/pull/31855#issuecomment-2957547018
(and previous comment for examples of why this is useful).

Example before this change, or with the setting in its default state:


![image](https://github.com/user-attachments/assets/f974c849-7ebf-424e-9397-442a6cc2513b)

Example after this change with the setting set to `true`:


![image](https://github.com/user-attachments/assets/bd8934f5-bd9a-4f5a-b723-cd4b798d2c2c)

Release Notes:

- Git: Added `collapse_untracked_diff` setting to auto-collapse
untracked diffs

assets/settings/default.json

@@ -713,6 +713,10 @@
     //
     // Default: false
     "sort_by_path": false,
+    // Whether to collapse untracked files in the diff panel.
+    //
+    // Default: false
+    "collapse_untracked_diff": false,
     "scrollbar": {
       // When to show the scrollbar in the git panel.
       //