Add git remote set-branches

Add git remote set-branches

Add ‘git remote set-branches’ for changing the list of tracked refs
for a remote repository with one "porcelain-level" command.  This
complements the longstanding ‘git remote add --track’ option.

The interface is based on the ‘git remote set-url’ subcommand.

   git remote set-branches base --add C
   git remote set-branches base A B D
   git remote set-branches base --delete D; # not implemented

Suggested-by: martin f. krafft <madduck@debian.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>