Support '*' in the middle of a refspec

Support '*' in the middle of a refspec

In order to keep the requirements strict, each * has to be a full path
component, and there may only be one * per side. This requirement is
enforced entirely by check_ref_format(); the matching implementation
will substitute the whatever matches the * in the lhs for the * in the
rhs.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>