Migrate most callers of git-related worktree APIs to use the GitStore (#27225)

This is a pure refactoring PR that goes through all the git-related APIs
exposed by the worktree crate and minimizes their use outside that
crate, migrating callers of those APIs to read from the GitStore
instead. This is to prepare for evacuating git repository state from
worktrees and making the GitStore the new source of truth.

Other drive-by changes:

- `project::git` is now `project::git_store`, for consistency with the
other project stores
- the project panel's test module has been split into its own file

Release Notes:

- N/A

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/title_bar/src/title_bar.rs

@@ -513,21 +513,10 @@ impl TitleBar {
     }
 
     pub fn render_project_branch(&self, cx: &mut Context<Self>) -> Option<impl IntoElement> {
-        let entry = {
-            let mut names_and_branches =
-                self.project.read(cx).visible_worktrees(cx).map(|worktree| {
-                    let worktree = worktree.read(cx);
-                    worktree.root_git_entry()
-                });
-
-            names_and_branches.next().flatten()
-        };
+        let repository = self.project.read(cx).active_repository(cx)?;
         let workspace = self.workspace.upgrade()?;
-        let branch_name = entry
-            .as_ref()
-            .and_then(|entry| entry.branch())
-            .map(|branch| branch.name.clone())
-            .map(|branch| util::truncate_and_trailoff(&branch, MAX_BRANCH_NAME_LENGTH))?;
+        let branch_name = repository.read(cx).current_branch()?.name.clone();
+        let branch_name = util::truncate_and_trailoff(&branch_name, MAX_BRANCH_NAME_LENGTH);
         Some(
             Button::new("project_branch_trigger", branch_name)
                 .color(Color::Muted)