test: optionally test contrib in CI

test: optionally test contrib in CI

Recently it was reported that a topic merged to 'next' broke build
and test for contrib/subtree part of the system.

Instead of having those who run 'next' or 'master' to hit the build
and test breakage and report to us, make sure we notice breakages in
contrib/ area before they hit my tree at all, during their own
presubmit testing.

Signed-off-by: Junio C Hamano <gitster@pobox.com>