How to make top-level git to track all the files under another sub-directory git

Question

command sequence

mkdir topdir
mkdir another-git-directory
touch fileC
touch fileD
git add .
git commit -m "sub-dir init"
cd ..
touch fileA
touch fileB
git add .
git commit -m "top-dir init"
git ls-files
// now we can see that fileC and fileD is not tracked by top-level git //
git ls-files -o
// this would not show us the fileC and fileD as untracked files//

my question is: how can we make "git ls-files -o" to show the untracked files inside sub-directory? why git is behaved such, as i expect git ls-files to show all untracked file (even it is inside another sub-dir git)?

I know that I could make the top git to track the sub-dir files using "git add */."... but I am interested to know why for the question above. Thanks!

directory structure

 topdir +  
        +-- .git  
        +-- fileA  
        +-- fileB  
        + another-git-directory +-- .git  
                                +-- fileC  
                                +-- fileD  

update (26 Jun)

I found this thread Unable to track files within Git submodules and this url (cn) http://blog.ossxp.com/2010/01/425/ which explains how to resolve the problem.

the solution:

git rm --cached another-git-directory #no trailing slash
git add another-git-directory/.
git commit

the 'git rm --cached path/to/sub-dir-or-sub-module' will tell the top-dir not to treat the sub-dir as a submodule... I think....

Answer 1

A nested repo is by default untracked.
(actually it tracks the submodule root folder as a gitlink, special entry in the index)
Git will detect the nested .git repo, and the parent repo will delegate any file status to the nested one.

To show them tracked, you may have to:

• remove the gitlink: git rm --cached mysubmodule.
  Note the lack of trailing slash: mysubmodule represents the gitlink, while mysubmodule/ is the folder created by the checked out submodule.
• add it back to the index of the parent repo: git add mysubmodule (here, you can add or not a trailing slash, it does not matter)
• commit the new files added and present in mysubmodule.