t5540-http-test: shorten grep pattern

t5540-http-test: shorten grep pattern

On OS X, the grep pattern

    "\"OP .*/objects/$x2/X38_X40 HTTP/[.0-9]*\" 20[0-9] "

is too long ($x38 and $x40 represent 38 and 40 copies of [0-9a-f]) for
grep to handle.  In order to still be able to match this, use the sed
invocation to replace what we're looking for with a token.

Improved-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>