git: Allow choosing branch diff target in branch diff view (#56569)

### Summary

Added a branch diff base branch picker that shows the selected diff base
and prioritizes branches on the active branch’s remote. This also
generalizes the branch picker’s checkout vs. select only behavior so its
UI/search/rendering can be reused by a future git graph picker, and
makes branch diff drop stale in progress base update tasks when the base
or repository changes for faster response.

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 is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes  #54050

Release Notes:

- git: Allow choosing branch diff base in branch diff view

---------

Co-authored-by: Remco Smits <djsmits12@gmail.com>

crates/git_ui/src/project_diff.rs

@@ -1,4 +1,5 @@
 use crate::{
+    branch_picker,
     conflict_view::ConflictAddon,
     git_panel::{GitPanel, GitPanelAddon, GitStatusEntry},
     git_panel_settings::GitPanelSettings,
@@ -37,7 +38,10 @@ use settings::{Settings, SettingsStore};
 use std::any::{Any, TypeId};
 use std::sync::Arc;
 use theme::ActiveTheme;
-use ui::{DiffStat, Divider, KeyBinding, Tooltip, prelude::*, vertical_divider};
+use ui::{
+    CommonAnimationExt as _, DiffStat, Divider, KeyBinding, PopoverMenu, Tooltip, prelude::*,
+    vertical_divider,
+};
 use util::{ResultExt as _, rel_path::RelPath};
 use workspace::{
     CloseActiveItem, ItemNavHistory, SerializableItem, ToolbarItemEvent, ToolbarItemLocation,
@@ -440,6 +444,13 @@ impl ProjectDiff {
                         async |cx| Self::refresh(this, RefreshReason::StatusesChanged, cx).await
                     })
                 }
+                BranchDiffEvent::DiffBaseChanged => {
+                    this.pending_scroll.take();
+                    this._task = window.spawn(cx, {
+                        let this = cx.weak_entity();
+                        async |cx| Self::refresh(this, RefreshReason::StatusesChanged, cx).await
+                    })
+                }
             },
         );
 
@@ -1152,6 +1163,8 @@ impl Item for ProjectDiff {
 impl Render for ProjectDiff {
     fn render(&mut self, _window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
         let is_empty = self.multibuffer.read(cx).is_empty();
+        let is_loading = self.branch_diff.read(cx).is_tree_base_loading() || !self._task.is_ready();
+
         let is_branch_diff_view = matches!(self.diff_base(cx), DiffBase::Merge { .. });
 
         div()
@@ -1165,7 +1178,17 @@ impl Render for ProjectDiff {
             .items_center()
             .justify_center()
             .size_full()
-            .when(is_empty, |el| {
+            .when(is_empty && is_loading, |el| {
+                let rems = TextSize::Large.rems(cx);
+                el.child(
+                    Icon::new(IconName::LoadCircle)
+                        .size(IconSize::Custom(rems))
+                        .color(Color::Accent)
+                        .with_rotate_animation(3)
+                        .into_any_element(),
+                )
+            })
+            .when(is_empty && !is_loading, |el| {
                 let remote_button = if let Some(panel) = self
                     .workspace
                     .upgrade()
@@ -1685,6 +1708,15 @@ impl Render for BranchDiffToolbar {
         let focus_handle = project_diff.focus_handle(cx);
         let review_count = project_diff.read(cx).total_review_comment_count();
         let (additions, deletions) = project_diff.read(cx).calculate_changed_lines(cx);
+        let diff_base = project_diff.read(cx).diff_base(cx).clone();
+        let DiffBase::Merge { base_ref } = diff_base else {
+            return div();
+        };
+        let selected_base_ref = base_ref.clone();
+        let base_ref_label = format!("Base: {base_ref}");
+        let repository = project_diff.read(cx).branch_diff.read(cx).repo().cloned();
+        let workspace = project_diff.read(cx).workspace.clone();
+        let project_diff_for_picker = project_diff.downgrade();
 
         let is_multibuffer_empty = project_diff.read(cx).multibuffer.read(cx).is_empty();
         let is_ai_enabled = AgentSettings::get_global(cx).enabled(cx);
@@ -1698,6 +1730,44 @@ impl Render for BranchDiffToolbar {
             .flex_wrap()
             .justify_end()
             .gap_2()
+            .child(
+                PopoverMenu::new("branch-diff-base-branch-picker")
+                    .menu(move |window, cx| {
+                        let project_diff = project_diff_for_picker.clone();
+                        let on_select =
+                            Arc::new(move |branch: git::repository::Branch, cx: &mut App| {
+                                let base_ref: SharedString = branch.name().to_owned().into();
+                                project_diff
+                                    .update(cx, |project_diff, cx| {
+                                        let branch_diff = &mut project_diff.branch_diff;
+                                        branch_diff.update(cx, |branch_diff, cx| {
+                                            branch_diff
+                                                .set_diff_base(DiffBase::Merge { base_ref }, cx);
+                                        });
+                                        cx.notify();
+                                    })
+                                    .ok();
+                            });
+                        Some(branch_picker::select_popover(
+                            workspace.clone(),
+                            repository.clone(),
+                            Some(selected_base_ref.clone()),
+                            on_select,
+                            window,
+                            cx,
+                        ))
+                    })
+                    .trigger_with_tooltip(
+                        Button::new("branch-diff-base-branch", base_ref_label)
+                            .color(Color::Muted)
+                            .end_icon(
+                                Icon::new(IconName::ChevronDown)
+                                    .size(IconSize::XSmall)
+                                    .color(Color::Muted),
+                            ),
+                        Tooltip::text("Select base branch"),
+                    ),
+            )
             .when(!is_multibuffer_empty, |this| {
                 this.child(DiffStat::new(
                     "branch-diff-stat",