Improve git-prune -n output

Improve git-prune -n output

prune_object() in show_only mode would previously just show the path to the
object that would be deleted.  The path the object is stored in shouldn't be
shown to users, they only know about sha1 identifiers so show that instead.

Further, the sha1 alone isn't that useful for examining what is going to be
deleted.  This patch also adds the object type to the output, which makes it
easy to pick out, say, the commits and use git-show to display them.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>