receive-pack: receive.denyDeleteCurrent

receive-pack: receive.denyDeleteCurrent

This is a companion patch to the recent 3d95d92 (receive-pack: explain
what to do when push updates the current branch, 2009-01-31).

Deleting the current branch from a remote will result in the next clone
from it not check out anything, among other things.  It also is one of the
cause that makes remotes/origin/HEAD a dangling symbolic ref.  This patch
still allows the traditional behaviour but with a big warning, and promises
that the default will change to 'refuse' in a future release.

Signed-off-by: Junio C Hamano <gitster@pobox.com>