Merge commit messages for batched pull requests

When the merge queue lands a batch with the merge-batch or fast-forward merge method, it writes merge commits on your base branch: one for each pull request and, with merge-batch, one for the whole batch. They are what git log --first-parent and GitHub’s commit list show, what GitHub shows as each pull request’s merge commit, and what you revert when something goes wrong. Until now they said Merged #42, #43 or Merge of #42 and nothing else, whatever you had configured. They now read like merges you made yourself:

• The batch commit says what landed: with merge-batch, it reads Merge #42, #43: <title of #42> (+1 more), and its body lists every pull request it lands with its title and author.
• Your format applies: the queue rule’s commit_message_format sets the title, the body and the trailers of each pull request’s merge commit, as it already did for the merge and squash methods.
• GitHub’s settings otherwise: without a format, Mergify follows your repository’s merge commit settings on GitHub, which by default give Merge pull request #42 from owner/branch and the pull request title.