WIP: Remove the --force option from gitlab-shell fetch-remote

The man page for git fetch says:

When git fetch is used with : refspec, it refuses to update the local branch unless the remote branch it fetches is a descendant of . This option overrides that check.

Since we never pass any refspecs to git fetch, this option is a no-op and can be safely removed.

This is preparatory work for #95 (closed) - since it doesn't need to support the option, it makes sense if it's removed.

We'll need an MR against gitlab-ce and gitlab-ee to remove the option there as well, if we agree this is a good idea.