t: add test for git refs optimize subcommand

Add a test script, `t/t1463-refs-optimize.sh`, for the new `git refs
optimize` command.

This script acts as a simple driver, leveraging the shared test library
created in the preceding commit. It works by overriding the
`$pack_refs` variable to "refs optimize" and then sourcing the
shared library (`t/pack-refs-tests.sh`).

This approach ensures that `git refs optimize` is tested against the
entire comprehensive test suite of `git pack-refs`, verifying
that it acts as a compatible drop-in replacement.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>