completion: silence pseudoref existence check

completion: silence pseudoref existence check

In 44dbb3bf29 (completion: support pseudoref existence checks for
reftables, 2023-12-19), we have extended the Bash completion script to
support future ref backends better by using git-rev-parse(1) to check
for pseudo-ref existence. This conversion has introduced a bug, because
even though we pass `--quiet` to git-rev-parse(1) it would still output
the resolved object ID of the ref in question if it exists.

Fix this by redirecting its stdout to `/dev/null` and add a test that
catches this behaviour. Note that the test passes even without the fix
for the "files" backend because we parse pseudo refs via the filesystem
directly in that case. But the test will fail with the "reftable"
backend.

Helped-by: Jeff King <peff@peff.net>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>