remote: Allow for in-memory remotes in UpdateRemoteMirror

As we're about to phase out usage of on-disk remotes, all RPCs which interact with them need to be converted to use in-memory remotes. UpdateRemoteMirror() doesn't yet have the ability to use them yet.

Implement support for in-memory remotes by configuring the remote via a set of Git configuration environment variables. Until downstream callers have been converted from on-disk to in-memory remotes both ways will stay supported.

Fixes #3649 (closed)