hook: mark non-parallelizable hooks

hook: mark non-parallelizable hooks

Several hooks are known to be inherently non-parallelizable, so initialize
them with RUN_HOOKS_OPT_INIT_FORCE_SERIAL. This pins jobs=1 and overrides
any hook.jobs or runtime -j flags.

These hooks are:
applypatch-msg, pre-commit, prepare-commit-msg, commit-msg, post-commit,
post-checkout, and push-to-checkout.

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