Project commit graph traversal

dotnet-authors should allow the user to set some anchor commit (such as `HEAD`) and enumerate/traverse all the commits in some order.

- Go through all the commits, starting from an anchor point
- Process up to N each round (to not process all at once normally)
- Initial front = the anchor commit
- Get its parents, front = parents
- Enumerate the front in some order (say by author/committer date)
- Multiple modes should be available
- Order the commits from the current front, then pick the first one — give it a number, pick its parents, add to the front
- This allows us to set up open and closed ranges of commits via picking the frontal commits for each boundary — say [(commit1, commit2); commit3] is a fully-defined range