Merge branch 'sb/submodule-clone-rr'

"git clone --resurse-submodules --reference $path $URL" is a way to
reduce network transfer cost by borrowing objects in an existing
$path repository when cloning the superproject from $URL; it
learned to also peek into $path for presense of corresponding
repositories of submodules and borrow objects from there when able.

* sb/submodule-clone-rr:
  clone: recursive and reference option triggers submodule alternates
  clone: implement optional references
  clone: clarify option_reference as required
  clone: factor out checking for an alternate path
  submodule--helper update-clone: allow multiple references
  submodule--helper module-clone: allow multiple references
  t7408: merge short tests, factor out testing method
  t7408: modernize style

diff --cc Documentation/config.txt
Simple merge

diff --cc builtin/submodule--helper.c
index e79790f,a366757..9d79f19
--- 1/builtin/submodule--helper.c
--- 2/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@@ -442,9 -442,10 +442,9 @@@ static int module_name(int argc, const 
  }
  
  static int clone_submodule(const char *path, const char *gitdir, const char *url,
-                          const char *depth, const char *reference, int quiet)
+                          const char *depth, struct string_list *reference, int quiet)
  {
 -      struct child_process cp;
 -      child_process_init(&cp);
 +      struct child_process cp = CHILD_PROCESS_INIT;
  
        argv_array_push(&cp.args, "clone");
        argv_array_push(&cp.args, "--no-checkout");

diff --cc cache.h
Simple merge

diff --cc git-submodule.sh
Simple merge

diff --cc sha1_file.c
Simple merge