Since it can be annoying to manually cleanup 40 tracking branches
which were removed by the remote system, 'git remote prune <n>'
can now be used to delete any tracking branches under <n> which
are no longer available on the remote system.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <junkio@cox.net>