submodule: allow submodule directory in gitignore

[Aleksey Komarov <leeeeha@gmail.com>]

Hi all!

I'm sorry if the letter came twice. I have troubles with my post client.

I want to organize my repository so its submodules would be located at the root
of repository. I'm trying to create .gitignore to ignore all files and don't
ignore directories at the same time:

$ cat .gitignore
*
!*/

Now, I'm trying to add a submodule to my repository, but fail to understand why
my .gitignore prevents it from being added. I use the following command to check
if my submodule will be ignored or not:

$ git add --dry-run --ignore-missing c/

I have noticed that result of this check is different when directory c/ already
exists and when it still doesn't by the time of the check.
The described behavior is illustrated by the following example:

$ mkdir git_test
$ cd git_test
$ git init
Initialized empty Git repository in D:/temp/git_test/.git/
$ echo \* >> .gitignore
$ echo \!\*\/ >> .gitignore
$ git add --dry-run --ignore-missing c/
The following paths are ignored by one of your .gitignore files:
c/
Use -f if you really want to add them.
$ mkdir c
$ git add --dry-run --ignore-missing c/
$

Is this behavior expected? If yes, where can I read about it? Which part of the
Git documentation should I consult?
How should I correct my .gitignore to pass this check in both cases, when
directory c/ exists and when it doesn't?