t7900: fix flaky test due to leaking background job

t7900: fix flaky test due to leaking background job

One of the recently-added tests in t7900 exercises git-maintanance(1)
with the `--detach` flag, which causes it to perform maintenance in the
background. We do not wait for the backgrounded process to exit though,
which causes the process to leak outside of the test, leading to racy
behaviour.

Fix this by synchronizing with the process via a separate file
descriptor. This is the same workaround as we use in t6500, see the
function `run_and_wait_for_auto_gc ()`.

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