[RFC PATCH 0/5] Demonstrate new parallel threading API

This series build on top of origin/sb/submodule-helper. The first patch is a fixup to the last commit in the target branch to fix a memory leak. The patch is not really part of the series, but as I chose to build on top of that series I can fix it up as we go.

The patch 2 adds a new API to easily use a threaded work pool.

patch 3 adds the command `submodule foreach_parallel` (completely untested, RFC!) which was the original goal of this series. But oh well, I got tricked by Peff to prove how awesome the new API for parallel threading is, so that's shown off in patches 4 and 5.

Note: Both patch 4 and 5 delete more lines of code than they add, improving 
readability a lot as you can focus on the actual task and not on the threading
stuff.

Any feedback welcome! Thanks, Stefan

Stefan Beller (5):
  FIXUP submodule: implement `module_clone` as a builtin helper
  thread-utils: add a threaded task queue
  submodule: helper to run foreach in parallel
  index-pack: Use the new worker pool
  pack-objects: Use new worker pool

 builtin/index-pack.c        |  71 ++++++-------
 builtin/pack-objects.c      | 175 +++++++++++---------------------
 builtin/submodule--helper.c | 159 +++++++++++++++++++++++++++--
 git-submodule.sh            |   9 ++
 run-command.c               |  29 +++---
 thread-utils.c              | 237 ++++++++++++++++++++++++++++++++++++++++++++
 thread-utils.h              |  40 ++++++++
 7 files changed, 545 insertions(+), 175 deletions(-)