hook: allow hooks to disable stdout_to_stderr

hook: allow hooks to disable stdout_to_stderr

The last batch of hooks converted to the hook.[ch] API introduced
a regression because pick_next_hook() always sets stdout_to_stderr
for its child processes.

Pre-push is the only hook API user which requires stdout_to_stderr
to be 0, so it can be argued that pre-push needs fixing, however
this will likely break many pre-push hooks, so it's better to allow
it to be 0, i.e. to match the previous behavior.

To prevent such regressions in the future, extend the hook tests to
verify hooks write to the expected stdout vs stderr streams and
maintain backward compatibility with the hooks output assumptions.

The tests are independent of the actual hook implementations: I've
tested they work the same before and after the hook.[ch] conversion
and will continue to work after we eventually introduce parallel
hook execution and config-based hooks.

Fixes: 3e2836a742d8 ("transport: convert pre-push to hook API")
Reported-by: Chris Darroch <chrisd@apache.org>
Suggested-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>