Cover letter

I've used Link: tag inside commit messages to link those to the cover letter, as I found there's quite often the need to look at the patchset to understand a patch. The cover letter, and the patch series give more context, even in the presence of a "good" commit message.

Those links wouldn't be needed if the cover letter was included as commit message for a merge, but, first, merge commit almost only happen as a result of a pull request, second, it's usual for a patchset to go through different trees.

Also, I've made my best to provide a changelog for each patch series, with links to previous iterations. Having the Link: to the cover letter allows access to this changelog.

Anyway, one could wonder when Git would be able to record the patchset history, so that this history is not recorded in mail box, but in proper version control.