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>

services/pull/pull.go

@@ -781,9 +781,15 @@ func GetSquashMergeCommitMessages(ctx context.Context, pr *issues_model.PullRequ
 		return "", err
 	}
 
-	mergeMessage := strings.TrimSpace(pr.Issue.Content) // use PR's title and description as squash commit message
-	if setting.Repository.PullRequest.PopulateSquashCommentWithCommitMessages {
-		mergeMessage = formatSquashMergeCommitMessages(limitedCommits) // use PR's commit messages as squash commit message
+	if err := pr.LoadBaseRepo(ctx); err != nil {
+		return "", err
+	}
+	var mergeMessage string
+	switch pr.BaseRepo.MustGetUnit(ctx, unit.TypePullRequests).PullRequestsConfig().DefaultSquashMergeMessage {
+	case repo_model.SquashMergeMessageCommitMessages:
+		mergeMessage = formatSquashMergeCommitMessages(limitedCommits)
+	case repo_model.SquashMergeMessagePRBody:
+		mergeMessage = strings.TrimSpace(pr.Issue.Content)
 	}
 	coAuthors := collectSquashMergeCommitCoAuthors(ctx, gitRepo, pr, headCommitRef, mergeBaseRef, limit, limitedCommits)
 	return buildSquashMergeCommitMessages(mergeMessage, coAuthors), nil
@@ -797,7 +803,7 @@ func buildSquashMergeCommitMessages(mergeMessage string, coAuthors []string) str
 	msgContent, msgSep, msgTrailer := git.CommitMessageSplitTrailer(mergeMessage)
 	if (msgSep == "" || msgSep == "\n\n") && msgTrailer == "" {
 		msgContent = strings.TrimRightFunc(msgContent, unicode.IsSpace)
-		msgSep = "\n\n---------\n\n"
+		msgSep = util.Iif(msgContent == "", "", "\n\n---------\n\n")
 	}
 	var sb strings.Builder
 	sb.WriteString(msgContent)

services/pull/pull_test.go

@@ -99,6 +99,7 @@ func TestBuildSquashMergeCommitMessages(t *testing.T) {
 		coAuthors []string
 		expected  string
 	}{
+		{"", []string{"the-user"}, "Co-authored-by: the-user\n"},
 		{"title", nil, "title"},
 		{"title", []string{"the-user"}, "title\n\n---------\n\nCo-authored-by: the-user\n"},
 		{"title\n\n", []string{"the-user"}, "title\n\n---------\n\nCo-authored-by: the-user\n"},