git remote update: Fallback to remote if group does not exist

git remote update: Fallback to remote if group does not exist

Previously, git remote update <remote> would fail unless there was
a remote group configured with the same name as the remote.
git remote update will now fall back to using the remote if no matching
group can be found.

This enables "git remote update -p <remote>..." to fetch and prune one
or more remotes, for example.

Signed-off-by: Finn Arne Gangstad <finnag@pvv.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>