bugreport: git checkout --recurse-submodules overwrites unstaged changes

[Petr Gregor <gregy@gregy.cz>]

Hello,
I believe we found a reproducible bug in git checkout
--recurse-submodules command. Documentation says, that this command
should not overwrite unstaged changes in submodules unless -f is
given. In reality local changes can be accidentally overwritten even
without -f flag.

I reproduced the issue with git version 2.20.1 like this:

git init
git submodule add https://github.com/Gregy/znapzend-debian submodule
git add . && git commit -m 'first'
git checkout -b 'newbranch'
(cd submodule && git checkout a3a7b0)
git add . && git commit -m 'set new branch to different submodule commit'
echo 'test' > submodule/debian/compat #create an unstaged change
git checkout master --recurse-submodules
#my change is overwritten

Thank you for maintaining git, it is awesome!

Petr Gregor