Fix "git remote update" with remotes.defalt set

Starting from commit 8db35596, "git remote update" (with no
group name given) will fail with the following message if
remotes.default has been set in the config file:

fatal: 'default' does not appear to be a git repository
fatal: The remote end hung up unexpectedly

The problem is that the --multiple option is not passed to
"git fetch" if no remote or group name is given on the command
line. Fix the problem by always passing the --multiple
option to "git fetch" (which actually simplifies the code).

Reported-by: YONETANI Tomokazu
Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>