enhance(pull)!: add per-repository default squash merge commit message (#39816)

Adds a repository setting for the default squash merge commit message
body with GitHub's options `PR_BODY`, `COMMIT_MESSAGES` and `BLANK`,
also settable via `squash_merge_commit_message` in the API. The commit
title stays the pull request title.

1. Breaking: API squash merges without `merge_message_field` now get the
same default message as the web merge box instead of a title-only
commit, which affects clients sending only `Do` like Renovate. An empty
value keeps title-only.
2. Breaking: `[repository.pull-request] DEFAULT_SQUASH_MERGE_MESSAGE`
replaces `POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES`, which still
works as a deprecated fallback. Repositories store the default when
created or next saved.
3. Deprecated app.ini keys now migrate through one table instead of
per-loader fallbacks, which also fixes the ignored `[mailer] CERT_FILE`
and `KEY_FILE` fallbacks. Deprecation warnings no longer claim a removal
version.

<img width="1891" height="571" alt="grafik"
src="https://github.com/user-attachments/assets/0fdcde13-1f11-4156-b598-d80eb0d04cbe"
/>

---------

Co-authored-by: silverwind <me@silverwind.io>

routers/api/v1/repo/pull.go

@@ -981,14 +981,23 @@ func MergePullRequest(ctx *context.APIContext) {
 	}
 
 	message := strings.TrimSpace(form.MergeTitleField)
-	if len(message) == 0 {
-		message, _, err = pull_service.GetDefaultMergeMessage(ctx, ctx.Repo.GitRepo, pr, repo_model.MergeStyle(form.Do))
+	useDefaultSquashBody := !form.HasMergeMessageField && !ctx.Req.Form.Has("merge_message_field") && repo_model.MergeStyle(form.Do) == repo_model.MergeStyleSquash
+	if len(message) == 0 || useDefaultSquashBody {
+		defaultTitle, defaultBody, err := pull_service.GetDefaultMergeMessage(ctx, ctx.Repo.GitRepo, pr, repo_model.MergeStyle(form.Do))
 		if err != nil {
 			ctx.APIErrorInternal(err)
 			return
 		}
+		message = util.IfZero(message, defaultTitle)
+		if useDefaultSquashBody {
+			squashCommitMessages, err := pull_service.GetSquashMergeCommitMessages(ctx, pr)
+			if err != nil {
+				ctx.APIErrorInternal(err)
+				return
+			}
+			form.MergeMessageField = git.CommitMessageMerge(squashCommitMessages, defaultBody)
+		}
 	}
-
 	form.MergeMessageField = strings.TrimSpace(form.MergeMessageField)
 	if len(form.MergeMessageField) > 0 {
 		message += "\n\n" + form.MergeMessageField

routers/api/v1/repo/repo.go

@@ -895,6 +895,10 @@ func updateRepoUnits(ctx *context.APIContext, opts api.EditRepoOption) error {
 			optional.AssignPtrValue(changed, &config.DefaultAllowMaintainerEdit, opts.DefaultAllowMaintainerEdit)
 			optional.AssignPtrString(changed, &config.DefaultMergeStyle, opts.DefaultMergeStyle)
 			optional.AssignPtrString(changed, &config.DefaultUpdateStyle, opts.DefaultUpdateStyle)
+			optional.AssignPtrString(changed, &config.DefaultSquashMergeMessage, opts.SquashMergeCommitMessage)
+			if opts.SquashMergeCommitMessage != nil && !config.DefaultSquashMergeMessage.IsValid() {
+				return util.NewInvalidArgumentErrorf("invalid squash_merge_commit_message: %s", config.DefaultSquashMergeMessage)
+			}
 			// only validate update-style fields when the caller is actually changing one of them,
 			// so unrelated PATCH calls don't reject historical configs.
 			if opts.AllowMergeUpdate != nil || opts.AllowRebaseUpdate != nil || opts.DefaultUpdateStyle != nil {