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git filter-branch --index-filter \
  'git ls-files -s | sed "s@[[:cntrl:]]\"*@&testSubModule/@" |
      GIT_INDEX_FILE=$GIT_INDEX_FILE.new \
        git update-index --index-info &&
  mv $GIT_INDEX_FILE.new $GIT_INDEX_FILE || true' HEAD

git push origin master

git pull origin master
git checkout --ours .
git commit -a

git remote add origin リモートリポジトリのパス(git@github.com:Xxx/testSubModule.git等)

cd testSubModule
git init
git add .
git commit -m "change to submodule"

git rm -r --cached testSubModule

git add testSubModule

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Your branch is up-to-date with 'origin/master'.
#
# Changes to be committed:
#       deleted:    .gitmodules
#       deleted:    testSubModule
#       new file:   testSubModule/README.md
#       new file:   testSubModule/...submoduleのファイルがたくさん...
#       new file:   testSubModule/...
#       new file:   testSubModule/...
#       new file:   testSubModule/...

rm -rf testSubModule/.git 

git reset --hard HEAD^

git push importSubmodule __work_for_import__

git filter-branch --subdirectory-filter testSubModule/ HEAD

git pull importSubmodule __work_for_import__

cd ..
git remote add importSubmodule testSubModule/

git checkout -b __work_for_import__

git rm .gitmodules 

git rm --cached testSubModule