git pw mbox-patch should support adding the Link: tag to the message

Given a suitable parameter to do so, git pw mbox-patch should add the Link: tag pointing at the patchwork URL (with message-id) to the mbox.

First, this is easier in git pw than in some script that actually applies the patch. Second, git pw can guarantee that such a link exists. (Currently dim just assumes the patchwork URL works, given a message. This is not necessarily the case for messages not sent to intel-gfx list.)