[Bug] With "[remotes] group = a b c", "git push group" does not work

... even though "git fetch group" does and acts as if we did "git fetch a; git fetch b; git fetch c".

Yes, I know it does not work, but I think it should.

Expose get_remote_group() and struct remote_group_data out of builtin/fetch.c, probably move them to remote.c, and have builtin/{fetch,push}.c use them, or something.

It probably is a good #leftoverbits microproject material, albeit on a bit larger side.