submodules: print "registered for path" message only once

submodules: print "registered for path" message only once

Since 2cd9de3e (submodule add: always initialize .git/config entry) the
message "Submodule '\$name' (\$url) registered for path '\$sm_path'" is
printed every time cmd_init() is called, e.g. each time "git submodule
update" is used with the --init option.

This was not intended and leads to bogus output which can confuse users
and build systems. Apart from that the $url variable was not set after the
first run which did the actual initialization and only "()" was printed
in subsequent runs where "($url)" was meant to inform the user about the
upstream repo.

Fix that by moving the say command in question into the if block where the
url is initialized, restoring the behavior that was in place before the
2cd9de3e commit. While at it also remove the comment which still describes
the logic used before 2cd9de3e and add a comment about how things work now.

Reported-by: Nicolas Viennot and Sid Nair <nicolas@viennot.com>
Reported-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>