t: add test for git refs exists subcommand

Add a test script, `t/t1462-refs-exists.sh`, for the `git refs exists` command. This script acts as a simple driver, leveraging the shared test library created in the preceding commit. It works by overriding the `$git_show_ref_exists` variable to "git refs exists" and then sourcing the shared library (`t/show-ref-exists-tests.sh`). This approach ensures that `git refs exists` is tested against the entire comprehensive test suite of `git show-ref --exists`, 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> Acked-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>