project: Show detached head commit SHA in branch pickers (#29007)

When Git is in a detached HEAD state, the branch is `None`, and we can't
get any meaningful information from it. This PR adds a `head_commit`
field to the snapshot, which is always populated with the HEAD details,
even when the branch is `None`.

This also pave path to fix:
https://github.com/zed-industries/zed/issues/28736

git panel branch picker (before, after):
<img width="197" alt="image"
src="https://github.com/user-attachments/assets/0b6abbba-2988-4890-a708-bcd8aad84f26"
/> <img width="198" alt="image"
src="https://github.com/user-attachments/assets/4b08b1a8-5e79-4aa3-a44e-932249602c18"
/>

title bar branch picker (before, after):
<img width="183" alt="image"
src="https://github.com/user-attachments/assets/d94357f8-a4da-4d60-8ddd-fdd978b99fdf"
/> <img width="228" alt="image"
src="https://github.com/user-attachments/assets/d20824a1-9279-44d6-afd1-bf9319fc50e4"
/>

Release Notes:

- Added head commit SHA information to the Git branch picker in the
title bar and Git panel.

crates/git_ui/src/git_panel.rs

@@ -2938,6 +2938,7 @@ impl GitPanel {
         let expand_tooltip_focus_handle = editor_focus_handle.clone();
 
         let branch = active_repository.read(cx).branch.clone();
+        let head_commit = active_repository.read(cx).head_commit.clone();
 
         let footer_size = px(32.);
         let gap = px(9.0);
@@ -2966,6 +2967,7 @@ impl GitPanel {
             .child(PanelRepoFooter::new(
                 display_name,
                 branch,
+                head_commit,
                 Some(git_panel.clone()),
             ))
             .child(
@@ -4291,6 +4293,8 @@ impl Render for GitPanelMessageTooltip {
 pub struct PanelRepoFooter {
     active_repository: SharedString,
     branch: Option<Branch>,
+    head_commit: Option<CommitDetails>,
+
     // Getting a GitPanel in previews will be difficult.
     //
     // For now just take an option here, and we won't bind handlers to buttons in previews.
@@ -4301,11 +4305,13 @@ impl PanelRepoFooter {
     pub fn new(
         active_repository: SharedString,
         branch: Option<Branch>,
+        head_commit: Option<CommitDetails>,
         git_panel: Option<Entity<GitPanel>>,
     ) -> Self {
         Self {
             active_repository,
             branch,
+            head_commit,
             git_panel,
         }
     }
@@ -4314,6 +4320,7 @@ impl PanelRepoFooter {
         Self {
             active_repository,
             branch,
+            head_commit: None,
             git_panel: None,
         }
     }
@@ -4339,11 +4346,26 @@ impl RenderOnce for PanelRepoFooter {
         const MAX_BRANCH_LEN: usize = 16;
         const MAX_REPO_LEN: usize = 16;
         const LABEL_CHARACTER_BUDGET: usize = MAX_BRANCH_LEN + MAX_REPO_LEN;
+        const MAX_SHORT_SHA_LEN: usize = 8;
 
-        let branch = self.branch.clone();
-        let branch_name = branch
+        let branch_name = self
+            .branch
             .as_ref()
-            .map_or(" (no branch)".into(), |branch| branch.name.clone());
+            .map(|branch| branch.name.clone())
+            .or_else(|| {
+                self.head_commit.as_ref().map(|commit| {
+                    SharedString::from(
+                        commit
+                            .sha
+                            .chars()
+                            .take(MAX_SHORT_SHA_LEN)
+                            .collect::<String>(),
+                    )
+                })
+            })
+            .unwrap_or_else(|| SharedString::from(" (no branch)"));
+        let show_separator = self.branch.is_some() || self.head_commit.is_some();
+
         let active_repo_name = self.active_repository.clone();
 
         let branch_actual_len = branch_name.len();
@@ -4449,7 +4471,7 @@ impl RenderOnce for PanelRepoFooter {
                         ),
                     )
                     .child(repo_selector)
-                    .when_some(branch.clone(), |this, _| {
+                    .when(show_separator, |this| {
                         this.child(
                             div()
                                 .text_color(cx.theme().colors().text_muted)