[BUG] Fail to add a module in a subdirectory if module is already cloned

[Jehan Bing <jehan@orb.com>]

Hi,

I'm getting an error if I try to add a module in a subdirectory and that 
module is already cloned.
Here are the steps to reproduce (git 1.7.8.3):

git init module
cd module
echo foo > foo
git add foo
git commit -m "init"
cd ..
git init super
cd super
echo foo > foo
git add foo
git commit -m "init"
git branch b1
git branch b2
git checkout b1
git submodule add ../module lib/module
git commit -m "module"
git checkout b2
rm -rf lib
git submodule add ../module lib/module

The last command returns:
     fatal: Not a git repository: ../.git/modules/lib/module
     Unable to checkout submodule 'lib/module'

The file lib/modules/.git contains:
     gitdir: ../.git/modules/lib/module
(missing an additional "../")

In branch b1, after adding the module, the file contained the full path:
     gitdir: /[...]/super/.git/modules/lib/module
Or contains the correct relative path after checking out b1 later:
     gitdir: ../../.git/modules/lib/module


Regards,
	Jehan