fix: explicitly specify main branch for hub client in migration utility (#522)

The current hub client does not pass revision into the argument, which
causes the moon-landing call to append `create_pr=1` query param to the
token API and returns 403 error.

Author: sirahd

data/src/migration_tool/migrate.rs

@@ -33,8 +33,14 @@ pub async fn migrate_with_external_runtime(
     repo_id: &str,
 ) -> Result<()> {
     let cred_helper = BearerCredentialHelper::new(hub_token.to_owned(), "");
-    let hub_client =
-        HubClient::new(hub_endpoint, RepoInfo::try_from(repo_type, repo_id)?, None, "xtool", "", cred_helper)?;
+    let hub_client = HubClient::new(
+        hub_endpoint,
+        RepoInfo::try_from(repo_type, repo_id)?,
+        Some("main".to_owned()),
+        "xtool",
+        "",
+        cred_helper,
+    )?;
 
     migrate_files_impl(file_paths, false, hub_client, cas_endpoint, None, false).await?;