t1500: ensure current --since= behavior remains

t1500: ensure current --since= behavior remains

This behavior of git-rev-parse is observed since git 1.8.3.1
at least(*), and likely earlier versions.

At least one git-reliant project in-the-wild relies on this
current behavior of git-rev-parse being able to handle multiple
--since= arguments without squeezing identical results together.
So add a test to prevent the potential for regression in
downstream projects.

(*) 1.8.3.1 the version packaged for CentOS 7.x

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>