Restore default GitHub merge commit messages for MERGE platform automerge

[lo1tuma]

Renovate 43.177.5 changed GitHub platform automerge to pass commitHeadline and commitBody to enablePullRequestAutoMerge:

#42056

That makes sense for SQUASH, where GitHub can otherwise use the full PR body as the squash commit body. But for MERGE, this changed the merge commit title from GitHub’s default:

Merge pull request #518 from lo1tuma/renovate/globals-17.x

to the Renovate commit message:

⬆️ Update dependency globals from 15.15.0 to 17.7.0 (#518)

This is effectively a breaking change for tools that rely on GitHub’s normal merge commit format. I use pr-log, a changelog generator based on GitHub pull requests, and it relied on the previous merge commit shape.

It also makes MERGE history noisier. For example, a history can now look like this:

a8f31c2 Add timeout handling for package downloads
3b7e9d4 Improve release notes formatting
91c4a6f Merge pull request #124 from example/fix-cache-invalidation
| 6d2f8a1 Fix cache invalidation after failed installs
4e9b0c7 Refine dependency graph pruning
c12d44a Update dependency left-pad from 1.2.0 to 1.3.0 (#125)
| f8a91be Update dependency left-pad from 1.2.0 to 1.3.0
7a6e2d9 Keep temporary package manifests out of snapshots
2d30f84 Document package resolution order

The left-pad merge commit and its child commit have almost identical messages, while the regular merge commit still has GitHub’s normal Merge pull request ... title.

---

Possible fixes:

1. Limit the change from fix(github): set commit message explicitly for platform automerge #42056 to SQUASH.

   This would keep the original fix where it is needed, while restoring the previous MERGE behavior.

2. Add a single config option for platform automerge commit messages.

   Example:

   {
     "platformAutomergeCommitMessage": "auto"
   }

   Possible values:

   auto
   platform-default
   commit-message
   

   Suggested behavior:

   auto: use Renovate's commit message for SQUASH, but use the platform default for MERGE
   platform-default: never pass commitHeadline or commitBody
   commit-message: current behavior
   

3. Add fine-grained strategy-specific control.

   Example:

   {
     "platformAutomergeCommitMessage": {
       "merge-commit": "platform-default",
       "squash": "commit-message",
       "rebase": "platform-default"
     }
   }

   A future extension could allow an explicit format:

   {
     "platformAutomergeCommitMessage": {
       "merge-commit": "platform-default",
       "squash": "{{{commitMessage}}} (#{{prNumber}})"
     }
   }

I do not feel strongly about the exact shape of the option. The important part is that users of GitHub MERGE platform automerge can keep GitHub’s default merge commit message.

[RahulGautamSingh]

Thanks for the detailed write-up, this is a real regression. The problem lies here:

// lib/modules/platform/github/index.ts
if (mergeMethod !== 'REBASE' && automergeCommitMessage) {
  // ...builds commitHeadline / commitBody
}

Passing commitHeadline/commitBody was only ever needed to stop GitHub from using the full PR body as the squash commit body (the point of #42056). For MERGE, overriding GitHub's default Merge pull request #N from ... title is an unintended side effect.

So Option 1 is the right fix i.e. scope the override to SQUASH only:

if (mergeMethod === 'SQUASH' && automergeCommitMessage) {

I'd skip Options 2/3. A dedicated config option can be added if a genuine need for per-strategy message control shows up later, that can be a separate feature.