I agree that a extension method would be handy. However, I fear that it might not be that discoverable. How about having both? BTW, `ReachableFrom` is an awesome name for the extension method!

@dahlbyk @carlosmn Can you think of a better name than `SubsetOfTheseReferencesThatCanReachAnyOfTheseCommits()` for the `ReferenceCollection` method.