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

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>