clone: delay cloning until after remote HEAD checking

clone: delay cloning until after remote HEAD checking

This gives us an opportunity to abort the command during remote HEAD
check without wasting much bandwidth.

Cloning with remote-helper remains before the check because the remote
helper updates mapped_refs, which is necessary for remote ref checks.
foreign_vcs field is used to indicate the transport is handled by
remote helper.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>