hook: allow pre-push parallel execution

hook: allow pre-push parallel execution

pre-push is the only hook that keeps stdout and stderr separate (for
backwards compatibility with git-lfs and potentially other users). This
prevents parallelizing it because run-command needs stdout_to_stderr=1
to buffer and de-interleave parallel outputs.

Since we now default to jobs=1, backwards compatibility is maintained
without needing any extension or extra config: when no parallelism is
requested, pre-push behaves exactly as before.

When the user explicitly opts into parallelism via hook.jobs > 1,
hook.<event>.jobs > 1, or -jN, they accept the changed output behavior.

Document this and let get_hook_jobs() set stdout_to_stderr=1 automatically
when jobs > 1, removing the need for any extension infrastructure.

Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>