update.sh: switch to using git fetch

update.sh: switch to using git fetch

Starting with Git version 1.6.6, the git remote update command was
re-implemented to spawn a git fetch subprocess to actually do the
update.

Since Girocco requires at least Git version 1.6.6 we can switch
from "git remote update" to "git fetch --multiple --all" instead.

Make the switch and use "git fetch --multiple" directly.

We do maintain the original "git remote update" semantics in that
if a "remotes.default" exists we fetch that instead of --all.

The advantage of switching is that now we can better control progress
output according to the $show_progress setting.

The "git remote update" command never learned the "--progress"
option, but the "git fetch" command did in version 1.7.1.

When running Git version 1.7.1 or later and $show_progress has been
requested explicitly pass the "--progress" option on to the
"git fetch" command to make the requested progress show up.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>