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'