ref-filter: add `short` modifier to 'tree' atom

ref-filter: add `short` modifier to 'tree' atom

Sometimes while using 'tree' atom, user might want to see abbrev hash
instead of full 40 character hash.

Just like 'objectname', it might be convenient for users to have the
`:short` and `:short=<length>` option for printing 'tree' hash.

Let's introduce `short` option to 'tree' atom.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>