I am trying to put the Eigen sources in another git repo. Eigen has a .gitignore file that includes core and core.* - probably do that core dumps don't end up in the repo. However, on Windows, this also ignores the Core folder, which contains important files.
How can I make git not ignore the Core folder, without changing the .gitignore file (which comes from the Eigen sources)
First, the libeigen/eigen/.gitignore rules should only affect subfolders/files from the Eigen folder.
Second, that repository does not have a Core folder. It does have a libeigen/eigen/Eigen/Core file though. Which would indeed be ignored on Windows.
Check that with:
git check-ignore -v eigen/Eigen/Core
Then, for testing, use the config core.ignorecase:
git -c core.ignorecase=false check-ignore -v eigen/Eigen/Core
Note that if you were to modify the Core file, you can force adding it to the index:
git add -f Core