Populate RepositoryBranch automatically

[MattKotsenas]

Following up from #188 and starting the discussion to populate RepositoryBranch automatically.

There are a few linked items in the old issue requesting this feature. Additionally, a new version of the DotNet.ReproducibleBuilds package was released, which sets this for some common CI providers (see dotnet/reproducible-builds#27). However, that has the side-effect of causing the nuspec to be different between a local build and a CI build (beyond what's already necessary).

I'm open to design discussions, but I'll propose an algorithm that's the same as the one currently provided by ReproducibleBuilds, with a fallback that's essentially git rev-parse --abbrev-ref HEAD (or similar) if no properties are provided.

[baronfel]

I think this would be a great thing to have, and SourceLink is a reasonable place to ship it. @tmat do you have thoughts here?

[tmat]

Sounds good to me. We'd definitely accept a PR.