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/fs/src/fake_git_repo.rs

@@ -10,7 +10,7 @@ use git::{
     },
     status::{FileStatus, GitStatus, StatusCode, TrackedStatus, UnmergedStatus},
 };
-use gpui::{AsyncApp, BackgroundExecutor};
+use gpui::{AsyncApp, BackgroundExecutor, SharedString};
 use ignore::gitignore::GitignoreBuilder;
 use rope::Rope;
 use smol::future::FutureExt as _;
@@ -491,4 +491,8 @@ impl GitRepository for FakeGitRepository {
     ) -> BoxFuture<'_, Result<String>> {
         unimplemented!()
     }
+
+    fn default_branch(&self) -> BoxFuture<'_, Result<Option<SharedString>>> {
+        unimplemented!()
+    }
 }