t5551: stop writing packed-refs directly

t5551: stop writing packed-refs directly

We have multiple tests in t5551 that write thousands of tags. To do so
efficiently we generate the tags by writing the `packed-refs` file
directly, which of course assumes that the reference database is backed
by the files backend.

Refactor the code to instead use a single `git update-ref --stdin`
command to write the tags. While the on-disk end result is not the same
as we now have a bunch of loose refs instead of a single packed-refs
file, the distinction shouldn't really matter for any of the tests that
use this helper.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>