ref-filter: introduce refname_atom_parser()

ref-filter: introduce refname_atom_parser()

Using refname_atom_parser_internal(), introduce refname_atom_parser()
which will parse the %(symref) and %(refname) atoms. Store the parsed
information into the 'used_atom' structure based on the modifiers used
along with the atoms.

Now the '%(symref)' atom supports the ':strip' atom modifier. Update the
Documentation and tests to reflect this.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Karthik Nayak <Karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>