remote set-head: set followRemoteHEAD to "warn" if "always"

remote set-head: set followRemoteHEAD to "warn" if "always"

When running "remote set-head" manually it is unlikely, that the user
would actually like to have "fetch" always update the remote/HEAD. On
the contrary, it is more likely, that the user would expect remote/HEAD
to stay the way they manually set it, and just forgot about having
"followRemoteHEAD" set to "always".

When "followRemoteHEAD" is set to "always" make running "remote
set-head" change the config to "warn".

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>