remote: fix "update [group...]"

remote: fix "update [group...]"

The rewrite in C inadvertently broke updating with remote groups: when you
pass parameters to "git remote update", it used to look up "remotes.<group>"
for every parameter, and interpret the value as a list of remotes to update.

Also, no parameter, or a single parameter "default" should update all
remotes that have not been marked with "skipDefaultUpdate".

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>