ref-filter: modify "%(objectname:short)" to take length

ref-filter: modify "%(objectname:short)" to take length

Add support for %(objectname:short=<length>) which would print the
abbreviated unique objectname of given length. When no length is
specified, the length is 'DEFAULT_ABBREV'. The minimum length is
'MINIMUM_ABBREV'. The length may be exceeded to ensure that the
provided object name is unique.

Add tests and documentation for the same.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Helped-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>