Teach BranchUpdater to set merge branch directly

[jamill]

This PR lets us set the upstream remote and merge branch onto a local branch directly.

This is useful when we are pushing a new local branch to a remote and want to set the upstream tracking info. Instead of performing the refspec transform to identify the name of the tracked branch in the refs/remotes namespace, we can set the merge branch directly (e.g., using the transforms exposed in #349)

[nulltoken]

Instead of exposing a method, how about exposing two properties (maybe RemoteName and MergeBranchName)?

[jamill]

hmm... and pass each one seperately through the branch updater? we could do it that way.

[dahlbyk]

You could define SetUpstreamTo() as an extension method for convenience, but expose all the possible mutations for BranchUpdater as properties.

[nulltoken]

You could define SetUpstreamTo() as an extension method for convenience

Actually, I'd rather not. At least, for now.

I'd like to try and settle a property based access only for xxxUpdaters in order to see how far we can stretch this out while keeping it usable. Adding methods in the mix may blur the picture a little bit.

[jamill]

Updated - I added two properties UpstreamRemote and UpstreamMergeBranch - I could change these to @nulltoken 's proposed names RemoteName and MergeBranchName if people feel that is more appropriate.

[nulltoken]

Updated - I added two properties UpstreamRemote and UpstreamMergeBranch

Perfect! And... merged 💥

[jamill]

@nulltoken Thanks!