hook: allow overriding the ungroup option

hook: allow overriding the ungroup option

When calling run_process_parallel() in run_hooks_opt(), the
ungroup option is currently hardcoded to .ungroup = 1.

This causes problems when ungrouping should be disabled, for
example when sideband-reading collated output from child hooks,
because sideband-reading and ungrouping are mutually exclusive.

Thus a new hook.h option is added to allow overriding.

The existing ungroup=1 behavior is preserved in the run_hooks()
API and the "hook run" command. We could modify these to take
an option if necessary, so I added two code comments there.

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