test-lib-functions: suppress a 'git rev-parse' error in 'test_commit_bulk'

When 'test_commit_bulk' is invoked in an empty test repository, it prints a "fatal: Needed a single revision" error, but still does what it's supposed to do. A test helper function displaying a fatal error and still succeeding is always suspect to be buggy, but luckily that's not the case here: that error comes from a 'git rev-parse --verify HEAD' command invoked in a condition, which doesn't have anything to verify in an empty repository. Use the '--quiet' option to suppress that error message. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Acked-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>