ref-filter: add deltabase option

ref-filter: add deltabase option

Add new formatting option: deltabase.
If the object is stored as a delta on-disk, this expands
to the 40-hex sha1 of the delta base object.
Otherwise, expands to the null sha1 (40 zeroes).
We have same option in cat-file command.
Hopefully, in the end I will remove formatting code from
cat-file and reuse formatting parts from ref-filter.

Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>