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