ref-filter: make remote_ref_atom_parser() use refname_atom_parser_internal()

ref-filter: make remote_ref_atom_parser() use refname_atom_parser_internal()

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>