Merge branch 'ar/parallel-hooks'

Merge branch 'ar/parallel-hooks'

Hook scripts defined via the configuration system can now be
configured to run in parallel.

* ar/parallel-hooks:
  t1800: test SIGPIPE with parallel hooks
  hook: allow hook.jobs=-1 to use all available CPU cores
  hook: add hook.<event>.enabled switch
  hook: move is_known_hook() to hook.c for wider use
  hook: warn when hook.<friendly-name>.jobs is set
  hook: add per-event jobs config
  hook: add -j/--jobs option to git hook run
  hook: mark non-parallelizable hooks
  hook: allow pre-push parallel execution
  hook: allow parallel hook execution
  hook: parse the hook.jobs config
  config: add a repo_config_get_uint() helper
  repository: fix repo_init() memleak due to missing _clear()