ref-filter: add support to sort by version

ref-filter: add support to sort by version

Add support to sort by version using the "v:refname" and
"version:refname" option. This is achieved by using the 'versioncmp()'
function as the comparing function for qsort.

This option is included to support sorting by versions in `git tag -l`
which will eventually be ported to use ref-filter APIs.

Add documentation and tests for the same.

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