release notes: one line per merged MR since the previous release #331

A read-only release notes <owner/name> <ref> [--since <tag>] lists the merge requests merged between the previous release's tag (or --since) and <ref>, one line each: - <title> (!<n>) by @<author>. The releases page gets a "Summarize changes" button on the New release form that fills the notes box with the same lines.

MRs are attributed by commit range: mr merge records the commit it lands (merged_sha, new column), falling back to head_sha for rows merged before the column existed. Old squash and rebase merges have no recoverable landed commit and are not listed.