In add_ref(), verify that the refname is formatted correctly before
adding it to the ref_list.  Here we have to allow refname components
that start with ".", since (for example) the remote protocol uses
synthetic reference name ".have".  So add a new REFNAME_DOT_COMPONENT
flag that can be passed to check_refname_format() to allow leading
dots.
Signed-off-by: NMichael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>