add support to custom main branch name
add support to custom main branch name

Summary: This diff adds support to customize main branch name when generating GitHub Actions.

Differential Revision: D30679305

fbshipit-source-id: 0fc7eb1c97c27e2b42e60cc1ab69a48ab93b93fa

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly