hook: run a list of hooks to prepare for multihook support

hook: run a list of hooks to prepare for multihook support

Hooks are limited to run one command (the default from the hookdir) for
each event. This limitation makes it impossible to run multiple commands
via config files, which the next commits will add.

Implement the ability to run a list of hooks in hook.[ch]. For now, the
list contains only one entry representing the "default" hook from the
hookdir, so there is no user-visible change in this commit.

All hook commands still run sequentially like before. A separate patch
series will enable running them in parallel.

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>