ref-filter: add new "describe" atom

ref-filter: add new "describe" atom

Duplicate the logic of %(describe) and friends from pretty to
ref-filter. In the future, this change helps in unifying both the
formats as ref-filter will be able to do everything that pretty is doing
and we can have a single interface.

The new atom "describe" and its friends are equivalent to the existing
pretty formats with the same name.

Helped-by: Junio C Hamano <redacted>
Mentored-by: Christian Couder <redacted>
Mentored-by: Hariom Verma <redacted>
Signed-off-by: Kousik Sanagavarapu <redacted>
Signed-off-by: Junio C Hamano <redacted>