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 <redacted>
Mentored-by: Matthieu Moy <redacted>
Helped-by: Jacob Keller <redacted>
Signed-off-by: Karthik Nayak <redacted>
Signed-off-by: Junio C Hamano <redacted>