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_02Signed-off-by: NEric Sunshine <sunshine@sunshineco.com>
Acked-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t7508-status.sh

@@ -61,7 +61,8 @@ test_expect_success 'status (1)' '
 '
 
 strip_comments () {
-	sed "s/^\# //; s/^\#$//; s/^#\t/\t/" <"$1" >"$1".tmp &&
+	tab='	'
+	sed "s/^\# //; s/^\#$//; s/^#$tab/$tab/" <"$1" >"$1".tmp &&
 	rm "$1" && mv "$1".tmp "$1"
 }