[Bug] Custom git-dir directory shouldn't be listed as “untracked”

Hi, I want to create a local repository with a custom dir name instead
of ".git", I used the env vars GIT_DIR and WORK_TREE:

export GIT_DIR=".customgitdir"
export GIT_WORK_TREE="."

Then I created a repo in an empty directory:

$ git init
Initialized empty Git repository in XXXXXXXXX/.customgitdir/

Then I ran git status:

$ git status
On branch master

Initial commit

Untracked files:
 (use "git add <file>..." to include in what will be committed)

   .customgitdir/

nothing added to commit but untracked files present (use "git add" to track)

The local repo directory listed as "untracked files" which is a
problem when using "git add ." afterwards.

When using the default directory ".git", it logically doesn't appear
in the "git status" command's output. Don't you think it should be the
same when using a custom dir name ?

Git version 2.6.4 on MacOSX 10.11

Regards,
Nicolas