Merge branch 'ab/hook-api-with-stdin'

Merge branch 'ab/hook-api-with-stdin'

Extend the run-hooks API to allow feeding data from the standard
input when running the hook script(s).

* ab/hook-api-with-stdin:
  hook: support a --to-stdin=<path> option
  sequencer: use the new hook API for the simpler "post-rewrite" call
  hook API: support passing stdin to hooks, convert am's 'post-rewrite'
  run-command: allow stdin for run_processes_parallel
  run-command.c: remove dead assignment in while-loop