submodules: ensure clean environment when operating in a submodule

git-submodule used to take care of clearing GIT_DIR whenever it operated
on a submodule index or configuration, but forgot to unset GIT_WORK_TREE
or other repo-local variables. This would lead to failures e.g. when
GIT_WORK_TREE was set.

This only happened in very unusual contexts such as operating on the
main worktree from outside of it, but since "git-gui: set GIT_DIR and
GIT_WORK_TREE after setup" (a9fa11f) such failures could also
be provoked by invoking an external tool such as "git submodule update"
from the Git Gui in a standard setup.

Solve by using the newly introduced clear_local_git_env() shell function
to ensure that all repo-local environment variables are unset.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -222,7 +222,7 @@ cmd_add()
 
 		module_clone "$path" "$realrepo" "$reference" || exit
 		(
-			unset GIT_DIR
+			clear_local_git_env
 			cd "$path" &&
 			# ash fails to wordsplit ${branch:+-b "$branch"...}
 			case "$branch" in
@@ -278,7 +278,7 @@ cmd_foreach()
 			name=$(module_name "$path")
 			(
 				prefix="$prefix$path/"
-				unset GIT_DIR
+				clear_local_git_env
 				cd "$path" &&
 				eval "$@" &&
 				if test -n "$recursive"
@@ -434,7 +434,7 @@ cmd_update()
 			module_clone "$path" "$url" "$reference"|| exit
 			subsha1=
 		else
-			subsha1=$(unset GIT_DIR; cd "$path" &&
+			subsha1=$(clear_local_git_env; cd "$path" &&
 				git rev-parse --verify HEAD) ||
 			die "Unable to find current revision in submodule path '$path'"
 		fi
@@ -454,7 +454,7 @@ cmd_update()
 
 			if test -z "$nofetch"
 			then
-				(unset GIT_DIR; cd "$path" &&
+				(clear_local_git_env; cd "$path" &&
 					git-fetch) ||
 				die "Unable to fetch in submodule path '$path'"
 			fi
@@ -477,22 +477,22 @@ cmd_update()
 				;;
 			esac
 
-			(unset GIT_DIR; cd "$path" && $command "$sha1") ||
+			(clear_local_git_env; cd "$path" && $command "$sha1") ||
 			die "Unable to $action '$sha1' in submodule path '$path'"
 			say "Submodule path '$path': $msg '$sha1'"
 		fi
 
 		if test -n "$recursive"
 		then
-			(unset GIT_DIR; cd "$path" && cmd_update $orig_args) ||
+			(clear_local_git_env; cd "$path" && cmd_update $orig_args) ||
 			die "Failed to recurse into submodule path '$path'"
 		fi
 	done
 }
 
 set_name_rev () {
 	revname=$( (
-		unset GIT_DIR
+		clear_local_git_env
 		cd "$1" && {
 			git describe "$2" 2>/dev/null ||
 			git describe --tags "$2" 2>/dev/null ||
@@ -757,7 +757,7 @@ cmd_status()
 		else
 			if test -z "$cached"
 			then
-				sha1=$(unset GIT_DIR; cd "$path" && git rev-parse --verify HEAD)
+				sha1=$(clear_local_git_env; cd "$path" && git rev-parse --verify HEAD)
 				set_name_rev "$path" "$sha1"
 			fi
 			say "+$sha1 $displaypath$revname"
@@ -767,7 +767,7 @@ cmd_status()
 		then
 			(
 				prefix="$displaypath/"
-				unset GIT_DIR
+				clear_local_git_env
 				cd "$path" &&
 				cmd_status $orig_args
 			) ||
@@ -818,7 +818,7 @@ cmd_sync()
 		if test -e "$path"/.git
 		then
 		(
-			unset GIT_DIR
+			clear_local_git_env
 			cd "$path"
 			remote=$(get_default_remote)
 			say "Synchronizing submodule url for '$name'"