Transfer manifest

[technoweenie]

This extracts the configured transfer adapters to an explicit Manifest type, instead of storing them on the go package itself. We get a few benefits here:

• Tests operate on immutable *config.Configuration and *transfer.Manifest objects. No resetting global state.
• The transfer configs are only loaded as needed.

I made sure that a manifest is loaded at most once per git-lfs command.

[sinbad]

Just need to update the comments in this area to point people at the new Manifest type to find registration functions etc

[sinbad]

👍 Great work. A couple of places where the manifest is re-created when I think it should be passed in if that's the principle we're going with; otherwise the manifest that is created in the command may be different to the one being used in for example the transfer queue. Doesn't affect anything now because manifests are only created as default from config so they match but could cause issues in future.

[technoweenie]

Thanks for pointing out my sloppy test and doc typos. I was having a hell of a time getting those custom tests to pass, and just forgot to re-enable them.

I'm going to leave the lfs.Environ() and TransferQueue changes to future refactoring PRs that will remove a reliance on config.Config.

[ttaylorr]

👍 indeed!