Add git-submodule command

This command can be used to initialize, update and inspect submodules. It
uses a .gitmodules file, readable by git-config, in the top level directory
of the 'superproject' to specify a mapping between submodule paths and
repository url.

Example .gitmodules layout:

[module "git"]
	url = git://git.kernel.org/pub/scm/git/git.git

With this entry in .gitmodules (and a commit reference in the index entry for
the path "git"), the command 'git submodule init' will clone the repository
at kernel.org into the directory "git".

Known issues
============
There is currently no way to override the url found in the .gitmodules file,
except by manually creating the subproject repository. The place to fix this
in the script has a rather long comment about a possible plan.

Funny paths will be quoted in the output from git-ls-files, but git-submodule
does not attempt to unquote (or even detect the presence of) such paths.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-submodule.sh

@@ -0,0 +1,194 @@
+#!/bin/sh
+#
+# git-submodules.sh: init, update or list git submodules
+#
+# Copyright (c) 2007 Lars Hjemli
+
+USAGE='[--quiet] [--cached] [status|init|update] [--] [<path>...]'
+. git-sh-setup
+require_work_tree
+
+init=
+update=
+status=
+quiet=
+cached=
+
+#
+# print stuff on stdout unless -q was specified
+#
+say()
+{
+	if test -z "$quiet"
+	then
+		echo "$@"
+	fi
+}
+
+#
+# Run clone + checkout on missing submodules
+#
+# $@ = requested paths (default to all)
+#
+modules_init()
+{
+	git ls-files --stage -- "$@" | grep -e '^160000 ' |
+	while read mode sha1 stage path
+	do
+		# Skip submodule paths that already contain a .git directory.
+		# This will also trigger if $path is a symlink to a git
+		# repository
+		test -d "$path"/.git && continue
+
+		# If there already is a directory at the submodule path,
+		# expect it to be empty (since that is the default checkout
+		# action) and try to remove it.
+		# Note: if $path is a symlink to a directory the test will
+		# succeed but the rmdir will fail. We might want to fix this.
+		if test -d "$path"
+		then
+			rmdir "$path" 2>/dev/null ||
+			die "Directory '$path' exist, but is neither empty nor a git repository"
+		fi
+
+		test -e "$path" &&
+		die "A file already exist at path '$path'"
+
+		url=$(GIT_CONFIG=.gitmodules git-config module."$path".url)
+		test -z "$url" &&
+		die "No url found for submodule '$path' in .gitmodules"
+
+		# MAYBE FIXME: this would be the place to check GIT_CONFIG
+		# for a preferred url for this submodule, possibly like this:
+		#
+		# modname=$(GIT_CONFIG=.gitmodules git-config module."$path".name)
+		# alturl=$(git-config module."$modname".url)
+		#
+		# This would let the versioned .gitmodules file use the submodule
+		# path as key, while the unversioned GIT_CONFIG would use the
+		# logical modulename (if present) as key. But this would need
+		# another fallback mechanism if the module wasn't named.
+
+		git-clone -n "$url" "$path" ||
+		die "Clone of submodule '$path' failed"
+
+		(unset GIT_DIR && cd "$path" && git-checkout -q "$sha1") ||
+		die "Checkout of submodule '$path' failed"
+
+		say "Submodule '$path' initialized"
+	done
+}
+
+#
+# Checkout correct revision of each initialized submodule
+#
+# $@ = requested paths (default to all)
+#
+modules_update()
+{
+	git ls-files --stage -- "$@" | grep -e '^160000 ' |
+	while read mode sha1 stage path
+	do
+		if ! test -d "$path"/.git
+		then
+			# Only mention uninitialized submodules when its
+			# path have been specified
+			test "$#" != "0" &&
+			say "Submodule '$path' not initialized"
+			continue;
+		fi
+		subsha1=$(unset GIT_DIR && cd "$path" &&
+			git-rev-parse --verify HEAD) ||
+		die "Unable to find current revision of submodule '$path'"
+
+		if test "$subsha1" != "$sha1"
+		then
+			(unset GIT_DIR && cd "$path" && git-fetch &&
+				git-checkout -q "$sha1") ||
+			die "Unable to checkout '$sha1' in submodule '$path'"
+
+			say "Submodule '$path': checked out '$sha1'"
+		fi
+	done
+}
+
+#
+# List all registered submodules, prefixed with:
+#  - submodule not initialized
+#  + different revision checked out
+#
+# If --cached was specified the revision in the index will be printed
+# instead of the currently checked out revision.
+#
+# $@ = requested paths (default to all)
+#
+modules_list()
+{
+	git ls-files --stage -- "$@" | grep -e '^160000 ' |
+	while read mode sha1 stage path
+	do
+		if ! test -d "$path"/.git
+		then
+			say "-$sha1 $path"
+			continue;
+		fi
+		revname=$(unset GIT_DIR && cd "$path" && git-describe $sha1)
+		if git diff-files --quiet -- "$path"
+		then
+			say " $sha1 $path ($revname)"
+		else
+			if test -z "$cached"
+			then
+				sha1=$(unset GIT_DIR && cd "$path" && git-rev-parse --verify HEAD)
+				revname=$(unset GIT_DIR && cd "$path" && git-describe $sha1)
+			fi
+			say "+$sha1 $path ($revname)"
+		fi
+	done
+}
+
+while case "$#" in 0) break ;; esac
+do
+	case "$1" in
+	init)
+		init=1
+		;;
+	update)
+		update=1
+		;;
+	status)
+		status=1
+		;;
+	-q|--quiet)
+		quiet=1
+		;;
+	--cached)
+		cached=1
+		;;
+	--)
+		break
+		;;
+	-*)
+		usage
+		;;
+	*)
+		break
+		;;
+	esac
+	shift
+done
+
+case "$init,$update,$status,$cached" in
+1,,,)
+	modules_init "$@"
+	;;
+,1,,)
+	modules_update "$@"
+	;;
+,,*,*)
+	modules_list "$@"
+	;;
+*)
+	usage
+	;;
+esac