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>

models/repo/git.go

@@ -39,6 +39,19 @@ const (
 	UpdateStyleRebase UpdateStyle = "rebase"
 )
 
+// SquashMergeMessage is the source of the default squash merge commit message body, values match GitHub's squash_merge_commit_message: https://docs.github.com/en/rest/repos/repos#update-a-repository
+type SquashMergeMessage string
+
+const (
+	SquashMergeMessageBlank          SquashMergeMessage = "BLANK"
+	SquashMergeMessageCommitMessages SquashMergeMessage = "COMMIT_MESSAGES"
+	SquashMergeMessagePRBody         SquashMergeMessage = "PR_BODY"
+)
+
+func (m SquashMergeMessage) IsValid() bool {
+	return m == SquashMergeMessageBlank || m == SquashMergeMessageCommitMessages || m == SquashMergeMessagePRBody
+}
+
 // UpdateDefaultBranch updates the default branch
 func UpdateDefaultBranch(ctx context.Context, repo *Repository) error {
 	_, err := db.GetEngine(ctx).ID(repo.ID).Cols("default_branch").Update(repo)

models/repo/repo_unit.go

@@ -131,6 +131,7 @@ type PullRequestsConfig struct {
 	DefaultUpdateStyle            UpdateStyle
 	DefaultDeleteBranchAfterMerge bool
 	DefaultMergeStyle             MergeStyle
+	DefaultSquashMergeMessage     SquashMergeMessage
 	DefaultAllowMaintainerEdit    bool
 	DefaultTargetBranch           string
 }
@@ -150,6 +151,7 @@ func DefaultPullRequestsConfig() *PullRequestsConfig {
 	cfg.DefaultDeleteBranchAfterMerge = setting.Repository.PullRequest.DefaultDeleteBranchAfterMerge
 	cfg.DefaultMergeStyle = MergeStyle(setting.Repository.PullRequest.DefaultMergeStyle)
 	cfg.DefaultMergeStyle = util.IfZero(cfg.DefaultMergeStyle, MergeStyleMerge)
+	cfg.DefaultSquashMergeMessage = SquashMergeMessage(setting.Repository.PullRequest.DefaultSquashMergeMessage)
 	return cfg
 }