jj-lib: git::fetch takes multiple remotes.

* Reimplement `git::fetch` in terms of the new lower-level
  `GitFetch` api, which allows more control over the stages
  of the fetch.
* Make `git::fetch` to accept multiple remotes, fetch from them
  all, before calling `import_refs`.
* Set `default_branch` to None in the `GitFetchStats` return value
  from `git::fetch`. 
* Update call sites to use new `git::clone` and `git::fetch`
* Update tests: One important side-effect to note (as demonstrated
  in the changed cli test) is that when there are multiple remotes,
  all fetches must pass before `import_refs` starts; no branch
  imports if a fetch from any remote fails. This is WAI.

Fixes: #4923