git remote update: New option --prune

git remote update: New option --prune

With the --prune (or -p) option, git remote update will also prune
all the remotes that it fetches.  Previously, you had to do a manual
git remote prune <remote> for each of the remotes you wanted to
prune, and this could be tedious with many remotes.

A single command will now update a set of remotes, and remove all
stale branches: git remote update -p [group]

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