[$] A policy for Link tags

The Git source-code management system stores a lot of information about
changes to code — but it does not hold everything that might be of interest
to 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 that
are not explained in the changelog. For some years now, many maintainers
have followed the convention of applying a Link tag to commits that points
back to the mailing-list posting of the change. Linus Torvalds has been
expressing his dislike for this convention for a while, though, and its
time appears to be coming to an end.