Git synthetic worktree

Hi,

I'd like to create a single worktree where files from multiple Git repositories (or refs) coexist side-by-side (no subfolder separation), but each with repository still independently tracked by Git.

There may be multiple uses cases such as vendoring where a child project constructs a synthetic worktree with all its dependencies. Another use case is to generate patches with a tool such as `Quilt <https://savannah.nongnu.org/projects/quilt>` where the ``patches`` folder is an independent repository.

For example:

``` PROJECT A ├ File A ├ File B └ README ```

``` PROJECT B ├ patches │ ├ File 1 │ └ File 2 └ README ```

``` SYNTHETIC ├ patches │ ├ File 1 │ └ File 2 ├ File A ├ File B └ README ```

To put it simply, I'd like to replicate `Overlay FS <https://docs.kernel.org/filesystems/overlayfs.html>` in Git.

Now what happens when we have the same file in the repositories? We can specify which repository is at the top of the stack.

The CLI experience could look like something like this:

``` $ git clone repoA $ git remote add repoB $ git worktree push repoB/main Usual git commands now applies to `repoB`. $ git worktree top repoA. Usual git commands now applies to `repoA`. $ git worktree pop repoB/main Files from `repoB` are gone. ```