hook: add jobs option

Allow the API callers to specify the number of jobs across which hook execution can be parallelized. It defaults to 1 and no hook currently changes it, so all hooks run sequentially as before. This allows us to both pave the way for parallel hook execution (that will be a follow-up patch series building upon this) and to finish the API conversion of builtin/receive-pack.c, keeping the output async sideband thread ("muxer") design as Peff suggested. When .jobs==1 nothing changes, the "copy_to_sideband" async thread still outputs directly via sideband channel 2, keeping the current (mostly) real-time output characteristics, avoids unnecessary poll delays or deadlock risks. When .jobs > 1, a more complex muxer is needed to buffer the hook output and avoid interleaving. After working on this mux I quickly realized I was re-implementing run-command with ungroup=0 so that idea was dropped in favor of run-command which outputs to stderr. In other words, run-command itself already can buffer/deinterleave pp child outputs (ungroup=0), so we can just connect its stderr to the sideband async task when jobs > 1. Maybe it helps to illustrate how it works with ascii graphics: [ Sequential (jobs = 1) ] [ Parallel (jobs > 1) ] +--------------+ +--------+ +--------+ | Hook Process | | Hook 1 | | Hook 2 | +--------------+ +--------+ +--------+ | | | | stderr (inherited) | stderr pipe | | | (captured) | v v v +-------------------------------------------------------------+ | Parent Process | | | | (direct write) [run-command (buffered)] | | | | | | | | writes | | v v | | +-------------------------------------------+ | | | stderr (FD 2) | | | +-------------------------------------------+ | | | | | | (dup2'd to pipe) | | v | | +-----------------------+ | | | sideband async thread | | | +-----------------------+ | +-------------------------------------------------------------+ When use_sideband == 0, the sideband async thread is missing, so this same architecture just outputs via the parent stderr stream. See the following commits for the hook API conversions doing this, using pre-existing sideband thread logic from `copy_to_sideband`. Suggested-by: Jeff King <peff@peff.net> Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>