Force git to treat a nested repository as a regular folder instead of submodule but still keep it as a git repository

This is an example directory structure

ParentRepo/
    .git/
    parent_file
    ChildRepo/
        .git/
        child_file
        child_untracked

I want ParentRepo to track changes of all files under itself (track parent_file, child_file, child_untracked, but don't track ChildRepo/.git) as if ChildRepo was not a git repository, but I need to keep ChildRepo as an git repository

There are three hard requirements

• The contents of ChildRepo must be tracked directly in ParentRepo and not as a reference to a commit of ChildRepo. e.g. I must be able to delete ChildRepo/ and recover it's contents at any point in the history of ParentRepo using only information from the ParentRepo/.git/ directory, without using an external copy of ChildRepo from another server or from backup. Adding ChildRepo as a submodule of ParentRepo is insufficient, since if the entire directory ChildRepo is deleted, there is no way to recover using only the information in ParentRepo. Recovery of ChildRepo/.git/ is not required
• ChildRepo must remain a git repo that has it's own development history independent of ParentRepo; we can not delete the ChildRepo/.git/ folder
• ParentRepo must be able to track changes that are never commited to ChildRepo e.g. child_untracked may not ever make it into the history of ChildRepo, but it should still be possible to track it from ParentRepo

Currently, git treats any directory with a .git/ subdirectory as a git repo and I need to bypass this behavior

These are the reasons for the weird conflicting requirements

• A complete build of any previous version must be possible, but the upstream copy of ChildRepo is not guaranteed to be available in the future. If ChildRepo is removed from ParentRepo in the future, it's possible to break a previous build that depends on ChildRepo because the contents are not stored in ParentRepo and upstream was deleted permanently. As for why ChildRepo can not be backed up separately, there is no way to enforce that a copy of ChildRepo must be stored together with ParentRepo for the entire future of ParentRepo, and there's also no guarantee that the copy of ChildRepo contains all commits referenced by ParentRepo if, for example, history of ChildRepo was rewritten. Preservation of previous builds is the primary goal and is of utmost importance here, therefore the parent repo must track the contents of all dependencies, not just the versions
• ChildRepo must track it's own development history independent of ParentRepo
• It doesn't always make sense, nor is it always possible to make a commit to ChildRepo at the exact moment I need to make a commit to ParentRepo

Untrack it as a submodule with git rm --cached, then re-add it as a subdirectory (or rather, add some individual files) using low-level tools:

git update-index --add ChildRepo/child_file

It used to be possible to simply do:

git add ChildRepo/

The trailing / is important, causing the contents rather than the repository to be added. Unfortunately, in current Git versions this seems to no longer work.

Note that you won't be able to add a .git subdirectory even if you tried – the exclusion is hardcoded. (But you could git fetch its branches into the parent to keep a local copy.)