t7900: exercise detaching via trace2 regions

In t7900, we exercise the `--detach` logic by checking whether the command ended up writing anything to its output or not. This supposedly works because we close stdin, stdout and stderr when daemonizing. But one, it breaks on platforms where daemonize is a no-op, like Windows. And second, that git-maintenance(1) outputs anything at all in these tests is a bug in the first place that we'll fix in a subsequent commit. Introduce a new trace2 region around the detach which allows us to more explicitly check whether the detaching logic was executed. This is a much more direct way to exercise the logic, provides a potentially useful signal to tracing logs and also works alright on platforms which do not have the ability to daemonize. Signed-off-by: Patrick Steinhardt <ps@pks.im> [jc: dropped a stale in-code comment from a test] Signed-off-by: Junio C Hamano <gitster@pobox.com>