t6003: uncomment test '--max-age=c3, --topo-order'

t6003: uncomment test '--max-age=c3, --topo-order'

Test '--max-age=c3, --topo-order' in t6003-rev-list-topo-order.sh has
been commented out as failing since its introduction in [1].  However,
the test is successful at least since commit [2] -- bisecting further is
harder because of incompatibility of such old Git code with modern
header file <openssl/bn.h> [3].

Uncomment this test to gain test coverage.

[1] f573571a21 ([PATCH] Add t/t6003 with some --topo-order tests,
    2005-07-07)
[2] 765ac8ec46 (Rip out merge-order and make "git log <paths>..." work
    again., 2006-02-28)
[3] BIGNUM used in git's `epoch.c` which was removed in [2] changed
    significantly between OpenSSL 1.0.2 and OpenSSL 1.1.0
    See also https://stackoverflow.com/a/42295243/1083697 and
    https://lore.kernel.org/git/Y71qiCs+oAS2OegH@coredump.intra.peff.net/

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>