t7900: exercise detaching via trace2 regions

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>