Align RepositoryBranch logic with .NET 9

[MattKotsenas]

Fixes #46

Addresses the difference in behavior between the .NET 9 SDK's version of SourceLink and our own branch logic. Doing so has two implications:

1. We no longer attempt to shorten git refs like refs/heads/ or refs/tags/
2. We prefer the tag over the branch over the PR ID (old logic was PR ID, tag, branch)

For code reviewers I suggest reviewing each commit separately, as the first moves the test data and covers all the combinations of values, and the second is the logic and test result change

[baronfel]

Thanks for the useful per-commit breakout! This LGTM.