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

submodule.h

@@ -32,7 +32,7 @@ void set_config_fetch_recurse_submodules(int value);
 void check_for_new_submodule_commits(unsigned char new_sha1[20]);
 int fetch_populated_submodules(const struct argv_array *options,
 			       const char *prefix, int command_line_option,
-			       int quiet);
+			       int quiet, int max_parallel_jobs);
 unsigned is_submodule_modified(const char *path, int ignore_untracked);
 int submodule_uses_gitfile(const char *path);
 int ok_to_remove_submodule(const char *path);