[$] A policy for Link tags

The Git source-code management system stores a lot of information aboutchanges to code - but it does not hold everything that might be of interestto a developer who needs to investigate a specific change in the future.Commits in a repository are the end result of a (sometimes extended)discussion; often, that discussion will result in changes to the code thatare not explained in the changelog. For some years now, many maintainershave followed the convention of applying a Link tag to commits that pointsback to the mailing-list posting of the change. Linus Torvalds has beenexpressing his dislike for this convention for a while, though, and itstime appears to be coming to an end.