Local clone

When given a path rather than a url, git bypasses the git-aware mechanisms and copies the odb files. Do that in our `git_clone` and export the function that does this.

As discussed in #1142

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This provides a way to override, like git, but I've no idea how to test that we've chosen the right function, as we can't peek into it. Maybe by making sure we don't call the transfer progress callback?