Merge branch 'ar/config-hooks'

Merge branch 'ar/config-hooks'

Allow hook commands to be defined (possibly centrally) in the
configuration files, and run multiple of them for the same hook
event.

* ar/config-hooks:
  hook: add -z option to "git hook list"
  hook: allow out-of-repo 'git hook' invocations
  hook: allow event = "" to overwrite previous values
  hook: allow disabling config hooks
  hook: include hooks from the config
  hook: add "git hook list" command
  hook: run a list of hooks to prepare for multihook support
  hook: add internal state alloc/free callbacks