git-clone: stop dumb protocol from copying refs outside heads/ and tags/.

git-clone: stop dumb protocol from copying refs outside heads/ and tags/.

Most notably, the original code first copied refs/remotes/ that
remote side had to local, and overwrote them by mapping refs/heads/
from the remote when a dumb protocol transport was used.

This makes the clone behaviour by dumb protocol in line with the
git native and rsync transports.

Signed-off-by: Junio C Hamano <junkio@cox.net>