git: Add option to branch from default branch in branch picker (#34663)

Closes #33700

The option shows up as an icon that appears on entries that would create
a new branch. You can also branch from the default by secondary
confirming, which the icon has a tooltip for as well.

We based the default branch on the results from this command: `git
symbolic-ref refs/remotes/upstream/HEAD` and fallback to `git
symbolic-ref refs/remotes/origin/HEAD`

Release Notes:

- Add option to create a branch from a default branch in git branch
picker

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/project/src/git_store.rs

@@ -4025,6 +4025,25 @@ impl Repository {
         })
     }
 
+    pub fn default_branch(&mut self) -> oneshot::Receiver<Result<Option<SharedString>>> {
+        let id = self.id;
+        self.send_job(None, move |repo, _| async move {
+            match repo {
+                RepositoryState::Local { backend, .. } => backend.default_branch().await,
+                RepositoryState::Remote { project_id, client } => {
+                    let response = client
+                        .request(proto::GetDefaultBranch {
+                            project_id: project_id.0,
+                            repository_id: id.to_proto(),
+                        })
+                        .await?;
+
+                    anyhow::Ok(response.branch.map(SharedString::from))
+                }
+            }
+        })
+    }
+
     pub fn diff(&mut self, diff_type: DiffType, _cx: &App) -> oneshot::Receiver<Result<String>> {
         let id = self.id;
         self.send_job(None, move |repo, _cx| async move {