git-submodule - Fix bugs in adding an existing repo as a module

git-submodule add would trip if path to the submodule included a space,
or if its .git was a gitdir: link to a GIT_DIR kept elsewhere. Fix both.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -167,8 +167,7 @@ cmd_add()
 	# perhaps the path exists and is already a git repo, else clone it
 	if test -e "$path"
 	then
-		if test -d "$path/.git" &&
-		test "$(unset GIT_DIR; cd $path; git rev-parse --git-dir)" = ".git"
+		if test -d "$path"/.git -o -f "$path"/.git
 		then
 			echo "Adding existing repo at '$path' to the index"
 		else