Index: use a bare repository rather than a non-bare repository without checkout

Ever since https://github.com/rust-lang/cargo/issues/4015 , Cargo avoids checking out the index as a working directory; however, cargo still creates the repository as a non-bare repository with a working directory that just doesn't have any files checked out.

Ideally, I'd love to store this as a bare repository instead, where git doesn't expect to have a working directory.

This would have a few advantages:

• Git tools would understand the repository better, making it easier to explore and understand and debug.
• Tools that want to check .cargo into git (for caching or testing or other purposes) would be able to do so; git can track a repo.git bare repository, but cannot track a .git directory.

(This was discussed briefly in a Cargo team meeting; I'm filing this issue as a follow-up.)

If we don't worry about a one-time re-download of the index, we could solve this by just switching to a bare repository.

Start by locating Cargo's index repository creation and checkout logic, then inspect any existing tests around index initialization. Compare the current non-bare setup with the bare-repository behavior described here, including the one-time re-download tradeoff. Done means the index is stored as a bare repository and relevant behavior is covered by tests.