hook: allow separate std[out|err] streams

hook: allow separate std[out|err] streams

The hook API assumes that all hooks merge stdout to stderr.

This assumption is proven wrong by pre-push: some of its users
actually expect separate stdout and stderr streams and merging
them will cause a regression.

Therefore this adds a mechanism to allow pre-push to separate
the streams, which will be used in the next commit.

The mechanism is generic via struct run_hooks_opt just in case
there are any more surprise exceptions like this.

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>