Bug: cherry-pick & submodule

["Ефимов Василий" <real@ispras.ru>]

I faced an unexpected behaviour during cherry-picking
a commit to a branch with a submodule.

Git graph:

A -- B [master]
  \
   `- C -- D [test]

Both branches have a file with name `a_file`.
It has been added by commit A.
Commits B and C add a folder with name `a_submodule` to the respective 
branches.
Commit C does it regularly by adding a file with name `a_submodule/content`.
Commit B adds a submodule with name `a_submodule`.
Commit D only modifies `a_file`.
Note that `a_file` and `a_submodule` are not related.

[repo]
   |- a_file
   `- a_submodule

When I trying to cherry pick commit D on commit B, I got
a conflict with `a_submodule`. Changes of `a_file` are
successfully cherry-picked.

I expected, that there would be no conflicts.

A bash script reproducing the bug is listed below.

Vasily.

====
rm -rf a_submodule a_repo

mkdir a_submodule
cd a_submodule
git init
touch a_file_in_a_submodule
git add a_file_in_a_submodule
git commit -m "add a file in a submodule"
cd ..

mkdir a_repo
cd a_repo
git init

touch a_file
git add a_file
git commit -m "add a file"

git branch test
git checkout test

mkdir a_submodule
touch a_submodule/content
git add a_submodule/content
git commit -m "add a regular folder with name a_submodule"

echo "123" > a_file
git add a_file
git commit -m "modify a file"

git checkout master

git submodule add ../a_submodule a_submodule
git submodule update a_submodule
git commit -m "add a submodule info folder with name a_submodule"

# Trying to cherry-pick modification of a file from test branch.
git cherry-pick test

# some debug
git status
====