git-arc: Cope with extra output

On my machine, for reasons beyond my understanding, I get this warning
two times when running arc:

Warning: Module "openssl" is already loaded in Unknown on line 0

so grep -v ^Warning: on all the arc call-conduit calls to filter them
out so we can properly parse the JSON with jq.

Sponsored by: Netflix
Reviewed by: markj, jhb
Differential Revision: https://reviews.freebsd.org/D36583