... 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.
