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>