问为什么‘`git子模块’每次都需要从远程存储库中获取？

EN

## make a new project
$> git --version
git version 1.7.5.4
$> mkdir new_proj; cd new_proj; git init
$> touch new_file_1.txt; touch new_file_2.txt
$> git add . && git commit -m "first commit"

## move into some development branch
$> git checkout -b cool_feature
$> <hack hack hack>
# in the middle, I add a submodule
$> git submodule add https://github.com/some/other_proj.git other_proj
$> git submodule update --init
$> ls -lR
new_file_1.txt
new_file_2.txt
other_proj
other_proj/that_file
other_proj/another_file

## I have to go back to master to do some work
$> git checkout master
# Why is other_proj still around?
$> git status
Untracked: other_proj
## Fine, I'll remove it, since I want a clean working copy, because I need to do some work and commits
$> git clean -f -d
$> <work work work>

## Now I'm ready to go back to cool_feature, but my submodules are empty!
$> git checkout cool_feature