Merge branch 'sb/submodule-parallel-update'

A major part of "git submodule update" has been ported to C to take
advantage of the recently added framework to run download tasks in
parallel.

* sb/submodule-parallel-update:
  clone: allow an explicit argument for parallel submodule clones
  submodule update: expose parallelism to the user
  submodule helper: remove double 'fatal: ' prefix
  git submodule update: have a dedicated helper for cloning
  run_processes_parallel: rename parameters for the callbacks
  run_processes_parallel: treat output of children as byte array
  submodule update: direct error message to stderr
  fetching submodules: respect `submodule.fetchJobs` config option
  submodule-config: drop check against NULL
  submodule-config: keep update strategy around

submodule-config.h

@@ -2,6 +2,7 @@
 #define SUBMODULE_CONFIG_CACHE_H
 
 #include "hashmap.h"
+#include "submodule.h"
 #include "strbuf.h"
 
 /*
@@ -14,6 +15,7 @@ struct submodule {
 	const char *url;
 	int fetch_recurse;
 	const char *ignore;
+	struct submodule_update_strategy update_strategy;
 	/* the sha1 blob id of the responsible .gitmodules file */
 	unsigned char gitmodules_sha1[20];
 };