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/repository.go

@@ -6,6 +6,7 @@ package setting
 import (
 	"os/exec"
 	"path/filepath"
+	"slices"
 	"strings"
 
 	"gitea.dev/modules/log"
@@ -94,7 +95,7 @@ var (
 			DefaultMergeMessageAllAuthors            bool
 			DefaultMergeMessageMaxApprovers          int
 			DefaultMergeMessageOfficialApproversOnly bool
-			PopulateSquashCommentWithCommitMessages  bool
+			DefaultSquashMergeMessage                string
 			AddCoCommitterTrailers                   bool
 			RetargetChildrenOnMerge                  bool
 			DelayCheckForInactiveDays                int
@@ -188,7 +189,7 @@ var (
 			DefaultMergeMessageAllAuthors            bool
 			DefaultMergeMessageMaxApprovers          int
 			DefaultMergeMessageOfficialApproversOnly bool
-			PopulateSquashCommentWithCommitMessages  bool
+			DefaultSquashMergeMessage                string
 			AddCoCommitterTrailers                   bool
 			RetargetChildrenOnMerge                  bool
 			DelayCheckForInactiveDays                int
@@ -206,7 +207,7 @@ var (
 			DefaultMergeMessageAllAuthors:            false,
 			DefaultMergeMessageMaxApprovers:          10,
 			DefaultMergeMessageOfficialApproversOnly: true,
-			PopulateSquashCommentWithCommitMessages:  false,
+			DefaultSquashMergeMessage:                "PR_BODY",
 			AddCoCommitterTrailers:                   true,
 			RetargetChildrenOnMerge:                  true,
 			DelayCheckForInactiveDays:                7,
@@ -339,6 +340,10 @@ func loadRepositoryFrom(rootCfg ConfigProvider) {
 		log.Fatal("Failed to map Repository.PullRequest settings: %v", err)
 	}
 
+	if !slices.Contains([]string{"BLANK", "COMMIT_MESSAGES", "PR_BODY"}, Repository.PullRequest.DefaultSquashMergeMessage) { // uppercase unlike other config values, to match GitHub's squash_merge_commit_message which the API also uses
+		log.Fatal("Invalid repository.pull-request.DEFAULT_SQUASH_MERGE_MESSAGE: %s", Repository.PullRequest.DefaultSquashMergeMessage)
+	}
+
 	if !rootCfg.Section("packages").Key("ENABLED").MustBool(Packages.Enabled) {
 		Repository.DisabledRepoUnits = append(Repository.DisabledRepoUnits, "repo.packages")
 	}