[PATCH] Add --reference option to git submodule

["Michael S. Tsirkin" <mst@redhat.com>]

This adds --reference option to 'git submodule add' and
'git submodule update' commands, which is passed on to git clone.
This option is useful when the submodule is large, such as linux kernel.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

I noticed that there's no easy way to specify a reference
for submodule repositories. Here's a patch to add this option.

 Documentation/git-submodule.txt |   14 ++++++++++++--
 git-submodule.sh                |   31 ++++++++++++++++++++++++++++---
 2 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-submodule.txt b/Documentation/git-submodule.txt
index 3b8df44..0584cb7 100644
--- a/Documentation/git-submodule.txt
+++ b/Documentation/git-submodule.txt
@@ -9,10 +9,12 @@ git-submodule - Initialize, update or inspect submodules
 SYNOPSIS
 --------
 [verse]
-'git submodule' [--quiet] add [-b branch] [--] <repository> <path>
+'git submodule' [--quiet] add [-b branch]
+	      [--reference <repository>] [--] <repository> <path>
 'git submodule' [--quiet] status [--cached] [--] [<path>...]
 'git submodule' [--quiet] init [--] [<path>...]
-'git submodule' [--quiet] update [--init] [-N|--no-fetch] [--] [<path>...]
+'git submodule' [--quiet] update [--init] [-N|--no-fetch]
+	      [--reference <repository>] [--] [<path>...]
 'git submodule' [--quiet] summary [--summary-limit <n>] [commit] [--] [<path>...]
 'git submodule' [--quiet] foreach <command>
 'git submodule' [--quiet] sync [--] [<path>...]
@@ -177,6 +179,14 @@ OPTIONS
 	This option is only valid for the update command.
 	Don't fetch new objects from the remote site.
 
+--reference <repository>::
+	This option is only valid for add and update commands.  These
+	commands sometimes need to clone a remote repository. In this case,
+	this option will be passed to linkgit:git-clone[1] command.
++
+*NOTE*: Do *not* use this option unless you have read the note
+for linkgit:git-clone[1] --reference and --shared options carefully.
+
 <path>...::
 	Paths to submodule(s). When specified this will restrict the command
 	to only operate on the submodules found at the specified paths.
diff --git a/git-submodule.sh b/git-submodule.sh
index 8e234a4..32be246 100755
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -15,6 +15,7 @@ require_work_tree
 command=
 branch=
 quiet=
+reference=
 cached=
 nofetch=
 
@@ -91,6 +92,7 @@ module_clone()
 {
 	path=$1
 	url=$2
+	reference="$3"
 
 	# If there already is a directory at the submodule path,
 	# expect it to be empty (since that is the default checkout
@@ -106,7 +108,12 @@ module_clone()
 	test -e "$path" &&
 	die "A file already exist at path '$path'"
 
-	git-clone -n "$url" "$path" ||
+	if test "reference"
+	then
+		git-clone "$reference" -n "$url" "$path"
+	else
+		git-clone -n "$url" "$path"
+	fi ||
 	die "Clone of '$url' into submodule path '$path' failed"
 }
 
@@ -131,6 +138,15 @@ cmd_add()
 		-q|--quiet)
 			quiet=1
 			;;
+		--reference)
+			case "$2" in '') usage ;; esac
+			reference="--reference=$2"
+			shift
+			;;
+		--reference=*)
+			reference="$1"
+			shift
+			;;
 		--)
 			shift
 			break
@@ -203,7 +219,7 @@ cmd_add()
 		git config submodule."$path".url "$url"
 	else
 
-		module_clone "$path" "$realrepo" || exit
+		module_clone "$path" "$realrepo" "$reference" || exit
 		(
 			unset GIT_DIR
 			cd "$path" &&
@@ -321,6 +337,15 @@ cmd_update()
 			shift
 			nofetch=1
 			;;
+		--reference)
+			case "$2" in '') usage ;; esac
+			reference="$2"
+			shift 2
+			;;
+		--reference=*)
+			reference="$1"
+			shift
+			;;
 		--)
 			shift
 			break
@@ -351,7 +376,7 @@ cmd_update()
 
 		if ! test -d "$path"/.git -o -f "$path"/.git
 		then
-			module_clone "$path" "$url" || exit
+			module_clone "$path" "$url" "$reference"|| exit
 			subsha1=
 		else
 			subsha1=$(unset GIT_DIR; cd "$path" &&
-- 
1.6.3.rc3.dirty