[PATCHv2] Add --reference option to git submodule.

Changes from v1: fixes in documentation, fix test usage and
make it portable.

 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..14256c6 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 the linkgit:git-clone[1] command.
++
+*NOTE*: Do *not* use this option unless you have read the note
+for linkgit:git-clone[1]'s --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..4989d86 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 -n "$reference"
+	then
+		git-clone "$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" &&