repo: write format ("git" or "local") to disk on init

We currently determine if the repo uses the Git backend or the local
backend by checking for presence of a `.jj/repo/store/git_target`
file. To make it easier to add out-of-tree backends, let's instead add
a file that indicates which backend to use.

lib/src/backend.rs

@@ -398,6 +398,10 @@ pub fn make_root_commit(empty_tree_id: TreeId) -> Commit {
 }
 
 pub trait Backend: Send + Sync + Debug {
+    /// A unique name that identifies this backend. Written to
+    /// `.jj/repo/store/backend` when the repo is created.
+    fn name(&self) -> &str;
+
     fn hash_length(&self) -> usize;
 
     fn git_repo(&self) -> Option<git2::Repository>;

lib/src/git_backend.rs

@@ -164,6 +164,10 @@ impl Debug for GitBackend {
 }
 
 impl Backend for GitBackend {
+    fn name(&self) -> &str {
+        "git"
+    }
+
     fn hash_length(&self) -> usize {
         HASH_LENGTH
     }

lib/src/local_backend.rs

@@ -110,6 +110,10 @@ fn not_found_to_backend_error(err: std::io::Error) -> BackendError {
 }
 
 impl Backend for LocalBackend {
+    fn name(&self) -> &str {
+        "local"
+    }
+
     fn hash_length(&self) -> usize {
         64
     }

lib/src/repo.rs

@@ -142,7 +142,10 @@ impl ReadonlyRepo {
     ) -> Arc<ReadonlyRepo> {
         let repo_path = repo_path.canonicalize().unwrap();
         ReadonlyRepo::init_repo_dir(&repo_path);
-        let store = Store::new(backend_factory(&repo_path.join("store")));
+        let store_path = repo_path.join("store");
+        let backend = backend_factory(&store_path);
+        fs::write(&store_path.join("backend"), backend.name()).unwrap();
+        let store = Store::new(backend);
         let repo_settings = user_settings.with_repo(&repo_path).unwrap();
         let op_store: Arc<dyn OpStore> = Arc::new(SimpleOpStore::init(repo_path.join("op_store")));
         let mut root_view = op_store::View::default();