t7508: avoid non-portable sed expression

2556b996 (status: disable display of '#' comment prefix by default;
2013-09-06) introduced tests which fail on Mac OS X due to unportable
use of \t (for TAB) in a sed expression. POSIX [1][2] also disallows
it. Fix this.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/sed.html#tag_20_116_13_02
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03_02

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>