cvsimport: add support for new style remote layout

cvsimport: add support for new style remote layout

cvsimport creates any branches found in the remote CVS repository
in the refs/heads namespace.  This makes sense for a repository
conversion.  When using git as a sane interface to a remote CVS
repository, that repository may well remain as the 'master'
respository.  In this model it makes sense to import the CVS
repository into the refs/remotes namespace.

Add a new option '-r <remote>' to set the remote name for
this import.  When this option is specified branches are named
refs/remotes/<remote>/branch, with HEAD named as master matching
git-clone separate remotes layout.  Without branches are placed
ion refs/heads, with HEAD named origin as before.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>