Merge branch 'bg/fetch-multi'

Merge branch 'bg/fetch-multi'

* bg/fetch-multi:
  Re-implement 'git remote update' using 'git fetch'
  builtin-fetch: add --dry-run option
  builtin-fetch: add --prune option
  teach warn_dangling_symref to take a FILE argument
  remote: refactor some logic into get_stale_heads()
  Add missing test for 'git remote update --prune'
  Add the configuration option skipFetchAll
  Teach the --multiple option to 'git fetch'
  Teach the --all option to 'git fetch'