git_ui: Add action to toggle diff base (#62966)

# Objective

- Make it easy to switch Git features between showing uncommitted
changes and all changes on the current branch.

## Solution

- Add `git::ToggleDiffBase`, which toggles the user setting between
`head` and `default_branch`.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

---

Release Notes:

- Git: Added an action to toggle the diff base between HEAD and the
default branch.

crates/git_ui/src/project_diff.rs

@@ -48,6 +48,8 @@ actions!(
         Diff,
         /// Shows working changes relative to HEAD.
         DiffHead,
+        /// Toggles the git diff base between HEAD and the default branch.
+        ToggleDiffBase,
         /// Adds files to the git staging area.
         Add,
         /// Opens a new agent thread with the branch diff for review.
@@ -77,6 +79,19 @@ impl ProjectDiff {
         workspace.register_action(|workspace, _: &DiffHead, window, cx| {
             Self::deploy_at(workspace, None, window, cx);
         });
+        workspace.register_action(|workspace, _: &ToggleDiffBase, _window, cx| {
+            settings::update_settings_file(
+                workspace.app_state().fs.clone(),
+                cx,
+                move |settings, _| {
+                    let git = settings.git.get_or_insert_default();
+                    git.diff_base = Some(match git.diff_base.unwrap_or_default() {
+                        GitDiffBaseSetting::Head => GitDiffBaseSetting::DefaultBranch,
+                        GitDiffBaseSetting::DefaultBranch => GitDiffBaseSetting::Head,
+                    });
+                },
+            );
+        });
         workspace.register_action(
             |workspace, _: &git_actions::ViewUncommittedChanges, window, cx| {
                 Self::deploy_at(workspace, None, window, cx);
@@ -1117,6 +1132,33 @@ mod tests {
             );
         });
 
+        cx.update(|window, cx| {
+            window.dispatch_action(ToggleDiffBase.boxed_clone(), cx);
+        });
+        cx.run_until_parked();
+        project.read_with(cx, |project, cx| {
+            assert_eq!(
+                project.git_store().read(cx).diff_base(),
+                GitDiffBaseSetting::DefaultBranch
+            );
+        });
+
+        cx.update(|window, cx| {
+            window.dispatch_action(ToggleDiffBase.boxed_clone(), cx);
+        });
+        cx.run_until_parked();
+        project.read_with(cx, |project, cx| {
+            assert_eq!(
+                project.git_store().read(cx).diff_base(),
+                GitDiffBaseSetting::Head
+            );
+        });
+
+        cx.update(|window, cx| {
+            window.dispatch_action(ToggleDiffBase.boxed_clone(), cx);
+        });
+        cx.run_until_parked();
+
         cx.update(|window, cx| {
             window.dispatch_action(Diff.boxed_clone(), cx);
         });