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

Documentation/fetch-options.txt

@@ -100,6 +100,13 @@ ifndef::git-pull[]
 	reference to a commit that isn't already in the local submodule
 	clone.
 
+-j::
+--jobs=<n>::
+	Number of parallel children to be used for fetching submodules.
+	Each will fetch from different submodules, such that fetching many
+	submodules will be faster. By default submodules will be fetched
+	one at a time.
+
 --no-recurse-submodules::
 	Disable recursive fetching of submodules (this has the same effect as
 	using the '--recurse-submodules=no' option).