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

strbuf.h

@@ -386,6 +386,12 @@ extern ssize_t strbuf_read_file(struct strbuf *sb, const char *path, size_t hint
  */
 extern int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint);
 
+/**
+ * Write the whole content of the strbuf to the stream not stopping at
+ * NUL bytes.
+ */
+extern ssize_t strbuf_write(struct strbuf *sb, FILE *stream);
+
 /**
  * Read a line from a FILE *, overwriting the existing contents of
  * the strbuf.  The strbuf_getline*() family of functions share