t0008: silence error in subshell when using `grep -v`

t0008: silence error in subshell when using `grep -v`

In t0008 we use `grep -v` in a subshell, but expect that this command
will sometimes not match anything. This would cause grep(1) to return an
error code, but given that we don't run with `set -e` we swallow this
error.

We're about to enable `set -e`. Prepare for this by ignoring any errors.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>