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/title_bar/src/title_bar.rs

@@ -46,6 +46,7 @@ pub use stories::*;
 
 const MAX_PROJECT_NAME_LENGTH: usize = 40;
 const MAX_BRANCH_NAME_LENGTH: usize = 40;
+const MAX_SHORT_SHA_LENGTH: usize = 8;
 
 const BOOK_ONBOARDING: &str = "https://dub.sh/zed-c-onboarding";
 
@@ -513,8 +514,23 @@ impl TitleBar {
     pub fn render_project_branch(&self, cx: &mut Context<Self>) -> Option<impl IntoElement> {
         let repository = self.project.read(cx).active_repository(cx)?;
         let workspace = self.workspace.upgrade()?;
-        let branch_name = repository.read(cx).branch.as_ref()?.name.clone();
-        let branch_name = util::truncate_and_trailoff(&branch_name, MAX_BRANCH_NAME_LENGTH);
+        let branch_name = {
+            let repo = repository.read(cx);
+            repo.branch
+                .as_ref()
+                .map(|branch| branch.name.clone())
+                .map(|name| util::truncate_and_trailoff(&name, MAX_BRANCH_NAME_LENGTH))
+                .or_else(|| {
+                    repo.head_commit.as_ref().map(|commit| {
+                        commit
+                            .sha
+                            .chars()
+                            .take(MAX_SHORT_SHA_LENGTH)
+                            .collect::<String>()
+                    })
+                })
+        }?;
+
         Some(
             Button::new("project_branch_trigger", branch_name)
                 .color(Color::Muted)