* GitFetch{} separates `fetch()` from `import_refs()`.
* This allows more control over the stages of the fetch so multiple fetches
can happen before `import_refs` resolves the changes in the local jj repo.
* Implement `git::fetch` in terms of the new api.
* Add a test case for initial fetch from a repo without `HEAD` set. This tests
the default branch retrieving behaviour.
Issue: #4923