opam repo set-url - add --no-action

[dra27]

At present opam repo set-url both changes the URL for the repo and issues an update. It would be useful (cf. ocurrent/docker-base-images#99 (comment)) to be able to have git remote set-url-like behaviour where the URL is updated but not yet fetched (although the default of actually updating is much more desirable than git's...)