Use the recently introduced refname_atom_parser_internal() within
remote_ref_atom_parser(), this provides a common base for all the ref
printing atoms, allowing %(upstream) and %(push) to also use the
':strip' option.
The atoms '%(push)' and '%(upstream)' will retain the ':track' and
':trackshort' atom modifiers to themselves as they have no meaning in
context to the '%(refname)' and '%(symref)' atoms.
Update the documentation and tests to reflect the same.
Signed-off-by: Karthik Nayak <Karthik.188@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>