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/forms/repo_form.go

@@ -133,6 +133,7 @@ type RepoSettingForm struct {
 	PullsAllowMergeUpdate            bool
 	PullsAllowRebaseUpdate           bool
 	PullsDefaultUpdateStyle          string
+	PullsDefaultSquashMergeMessage   string
 	DefaultDeleteBranchAfterMerge    bool
 	DefaultAllowMaintainerEdit       bool
 	DefaultTargetBranch              string
@@ -424,20 +425,21 @@ type MergePullRequestForm struct {
 	ForceMerge             bool   `json:"force_merge,omitempty"`
 	MergeWhenChecksSucceed bool   `json:"merge_when_checks_succeed,omitempty"`
 	DeleteBranchAfterMerge *bool  `json:"delete_branch_after_merge,omitempty"`
+	HasMergeMessageField   bool   `form:"-" json:"-"`
 }
 
 func (f *MergePullRequestForm) UnmarshalJSON(b []byte) error {
 	// This is for backward compatibility, to support both field names like "do" and "Do",
 	// because old code doesn't have "json" tag for these fields
 	type aux struct {
-		Do1                string `json:"do"`
-		Do2                string `json:"Do"`
-		MergeTitleField1   string `json:"merge_title_field"`
-		MergeTitleField2   string `json:"MergeTitleField"`
-		MergeMessageField1 string `json:"merge_message_field"`
-		MergeMessageField2 string `json:"MergeMessageField"`
-		MergeCommitID1     string `json:"merge_commit_id"`
-		MergeCommitID2     string `json:"MergeCommitID"`
+		Do1                string  `json:"do"`
+		Do2                string  `json:"Do"`
+		MergeTitleField1   string  `json:"merge_title_field"`
+		MergeTitleField2   string  `json:"MergeTitleField"`
+		MergeMessageField1 *string `json:"merge_message_field"`
+		MergeMessageField2 *string `json:"MergeMessageField"`
+		MergeCommitID1     string  `json:"merge_commit_id"`
+		MergeCommitID2     string  `json:"MergeCommitID"`
 
 		HeadCommitID           string `json:"head_commit_id"`
 		ForceMerge             bool   `json:"force_merge"`
@@ -450,7 +452,10 @@ func (f *MergePullRequestForm) UnmarshalJSON(b []byte) error {
 	}
 	f.Do = util.IfZero(a.Do1, a.Do2)
 	f.MergeTitleField = util.IfZero(a.MergeTitleField1, a.MergeTitleField2)
-	f.MergeMessageField = util.IfZero(a.MergeMessageField1, a.MergeMessageField2)
+	f.MergeMessageField, f.HasMergeMessageField = "", false
+	if mergeMessageField := util.IfZero(a.MergeMessageField1, a.MergeMessageField2); mergeMessageField != nil {
+		f.MergeMessageField, f.HasMergeMessageField = *mergeMessageField, true
+	}
 	f.MergeCommitID = util.IfZero(a.MergeCommitID1, a.MergeCommitID2)
 	f.HeadCommitID = a.HeadCommitID
 	f.ForceMerge = a.ForceMerge

services/forms/repo_form_test.go

@@ -51,6 +51,7 @@ func TestMergePullRequestForm(t *testing.T) {
 		ForceMerge:             true,
 		MergeWhenChecksSucceed: true,
 		DeleteBranchAfterMerge: new(true),
+		HasMergeMessageField:   true,
 	}
 
 	t.Run("NewFields", func(t *testing.T) {
@@ -84,4 +85,18 @@ func TestMergePullRequestForm(t *testing.T) {
 		require.NoError(t, json.Unmarshal([]byte(input), &m))
 		assert.Equal(t, expected, m)
 	})
+
+	t.Run("OmittedMergeMessageFieldClearsPreviousMessage", func(t *testing.T) {
+		form := &MergePullRequestForm{MergeMessageField: "previous message", HasMergeMessageField: true}
+		require.NoError(t, json.Unmarshal([]byte(`{"do": "squash"}`), &form))
+		assert.Empty(t, form.MergeMessageField)
+		assert.False(t, form.HasMergeMessageField)
+	})
+
+	t.Run("EmptyMergeMessageField", func(t *testing.T) {
+		var form MergePullRequestForm
+		require.NoError(t, json.Unmarshal([]byte(`{"do": "squash", "merge_message_field": ""}`), &form))
+		assert.Empty(t, form.MergeMessageField)
+		assert.True(t, form.HasMergeMessageField)
+	})
 }