ref-filter: make "%(symref)" atom work with the ':short' modifier

ref-filter: make "%(symref)" atom work with the ':short' modifier

The "%(symref)" atom doesn't work when used with the ':short' modifier
because we strictly match only 'symref' for setting the 'need_symref'
indicator. Fix this by comparing with the valid_atom rather than the
used_atom.

Add tests for %(symref) and %(symref:short) while we're here.

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