t9200: avoid grep on non-ASCII data

t9200: avoid grep on non-ASCII data

GNU grep 2.23 detects the input used in this test as binary data so it
does not work for extracting lines from a file.  We could add the "-a"
option to force grep to treat the input as text, but not all
implementations support that.  Instead, use sed to extract the desired
lines since it will always treat its input as text.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>