A "content store" abstraction.

This is where the file content is stored, away from the repo.

Also two implementations of the abstraction: a local one for testing
and a remote one for production use.
Actually it was intended to be a hint to implementers that they don't need to worry about writing to a temp file and doing an atomic rename, as the target I'm telling them to write to is already a temp file.

Can you think of a more evocative way of conveying that? Clearly this didn't work.