submodule init: redirect stdout to stderr

submodule init: redirect stdout to stderr

Reroute the output of stdout to stderr as it is just informative
messages, not to be consumed by machines.

This should not regress any scripts that try to parse the
current output, as the output is already internationalized
and therefore unstable.

We want to init submodules from the helper for `submodule update`
in a later patch and the stdout output of said helper is consumed
by the parts of `submodule update` which are still written in shell.
So we have to be careful which messages are on stdout.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>