t: add test for git refs list subcommand

Extract the test logic from `t/t6300-for-each-ref.sh` into a new helper
library `lib-for-each-ref.sh`, enabling reuse across multiple commands.

Update `t/t6300-for-each-ref.sh` to use GIT_WHICH_TEST='for-each-ref'
with the shared logic from the new library.

Add a new test script `t/t1461-refs-list.sh` that reuses the same logic
with GIT_WHICH_TEST='refs list' to validate the behavior of the new git
refs list subcommand.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: shejialuo <shejialuo@gmail.com>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Meet Soni <meetsoni3017@gmail.com>

Author: inosmeet