clone: add `--remote-submodules` flag

When using `git clone --recurse-submodules` there was previously no way to
pass a `--remote` switch to the implicit `git submodule update` command for
any use case where you want the submodules to be checked out on their
remote-tracking branch rather than with the SHA-1 recorded in the superproject.

This patch rectifies this situation. It actually passes `--no-fetch` to
`git submodule update` as well on the grounds they the submodule has only just
been cloned, so fetching from the remote again only serves to slow things down.

Signed-off-by: Ben Avison <bavison@riscosopen.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-clone.txt b/Documentation/git-clone.txt
index a0f14b5..5fc97f1 100644
--- a/Documentation/git-clone.txt
+++ b/Documentation/git-clone.txt
@@ -15,7 +15,8 @@
 	  [--dissociate] [--separate-git-dir <git dir>]
 	  [--depth <depth>] [--[no-]single-branch] [--no-tags]
 	  [--recurse-submodules[=<pathspec>]] [--[no-]shallow-submodules]
-	  [--jobs <n>] [--] <repository> [<directory>]
+	  [--[no-]remote-submodules] [--jobs <n>] [--] <repository>
+	  [<directory>]
 
 DESCRIPTION
 -----------
@@ -260,6 +261,12 @@
 --[no-]shallow-submodules::
 	All submodules which are cloned will be shallow with a depth of 1.
 
+--[no-]remote-submodules::
+	All submodules which are cloned will use the status of the submodule’s
+	remote-tracking branch to update the submodule, rather than the
+	superproject’s recorded SHA-1. Equivalent to passing `--remote` to
+	`git submodule update`.
+
 --separate-git-dir=<git dir>::
 	Instead of placing the cloned repository where it is supposed
 	to be, place the cloned repository at the specified directory,