Merge branch 'sb/submodule-parallel-fetch'

Add a framework to spawn a group of processes in parallel, and use
it to run "git fetch --recurse-submodules" in parallel.

Rerolled and this seems to be a lot cleaner.  The merge of the
earlier one to 'next' has been reverted.

* sb/submodule-parallel-fetch:
submodules: allow parallel fetching, add tests and documentation
fetch_populated_submodules: use new parallel job processing
run-command: add an asynchronous parallel child processor
sigchain: add command to pop all common signals
strbuf: add strbuf_read_once to read without blocking
xread: poll on non blocking fds
submodule.c: write "Fetching submodule <foo>" to stderr

diff --cc submodule.h
Simple merge

diff --cc wrapper.c
Simple merge