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>

modules/setting/git.go

@@ -88,16 +88,6 @@ func loadGitFrom(rootCfg ConfigProvider) {
 	GitConfig.SetOption("diff.algorithm", "histogram")
 	GitConfig.SetOption("core.logAllRefUpdates", "true")
 
-	secGitReflog := rootCfg.Section("git.reflog")
-	if secGitReflog.HasKey("ENABLED") {
-		deprecatedSetting(rootCfg, "git.reflog", "ENABLED", "git.config", "core.logAllRefUpdates", "1.21")
-		GitConfig.SetOption("core.logAllRefUpdates", secGitReflog.Key("ENABLED").In("true", []string{"true", "false"}))
-	}
-	if secGitReflog.HasKey("EXPIRATION") {
-		deprecatedSetting(rootCfg, "git.reflog", "EXPIRATION", "git.config", "gc.reflogExpire", "1.21")
-		GitConfig.SetOption("gc.reflogExpire", secGitReflog.Key("EXPIRATION").String()+".days")
-	}
-
 	for _, key := range secGitConfig.Keys() {
 		GitConfig.SetOption(key.Name(), key.String())
 	}