Merge branch 'ab/config-based-hooks-base' into seen

Merge branch 'ab/config-based-hooks-base' into seen

Restructuring of (a subset of) Emily's config-based-hooks series,
to demonstrate that a series can be presented as a more logical and
focused progression.

* ab/config-based-hooks-base: (30 commits)
  hook-list.h: add a generated list of hooks, like config-list.h
  hooks: fix a TOCTOU in "did we run a hook?" heuristic
  receive-pack: convert receive hooks to hook.h
  post-update: use hook.h library
  receive-pack: convert 'update' hook to hook.h
  hooks: allow callers to capture output
  run-command: allow capturing of collated output
  reference-transaction: use hook.h to run hooks
  transport: convert pre-push hook to use config
  hook: convert 'post-rewrite' hook in sequencer.c to hook.h
  hook: provide stdin by string_list or callback
  run-command: add stdin callback for parallelization
  am: convert 'post-rewrite' hook to hook.h
  hook: support passing stdin to hooks
  run-command: allow stdin for run_processes_parallel
  run-command: remove old run_hook_{le,ve}() hook API
  receive-pack: convert push-to-checkout hook to hook.h
  read-cache: convert post-index-change hook to use config
  commit: use hook.h to execute hooks
  git-p4: use 'git hook' to run hooks
  ...