Always use an empty line to separate the commit message and trailer (#34512)

If the message from form.MergeMessageField is empty, we will miss a "\n"
between the title and the "Co-authored-by:" line. The title and message
should have a blank line between of them.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/pull/merge.go

@@ -13,6 +13,7 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"unicode"
 
 	"code.gitea.io/gitea/models/db"
 	git_model "code.gitea.io/gitea/models/git"
@@ -161,6 +162,41 @@ func GetDefaultMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr
 	return getMergeMessage(ctx, baseGitRepo, pr, mergeStyle, nil)
 }
 
+func AddCommitMessageTailer(message, tailerKey, tailerValue string) string {
+	tailerLine := tailerKey + ": " + tailerValue
+	message = strings.ReplaceAll(message, "\r\n", "\n")
+	message = strings.ReplaceAll(message, "\r", "\n")
+	if strings.Contains(message, "\n"+tailerLine+"\n") || strings.HasSuffix(message, "\n"+tailerLine) {
+		return message
+	}
+
+	if !strings.HasSuffix(message, "\n") {
+		message += "\n"
+	}
+	pos1 := strings.LastIndexByte(message[:len(message)-1], '\n')
+	pos2 := -1
+	if pos1 != -1 {
+		pos2 = strings.IndexByte(message[pos1:], ':')
+		if pos2 != -1 {
+			pos2 += pos1
+		}
+	}
+	var lastLineKey string
+	if pos1 != -1 && pos2 != -1 {
+		lastLineKey = message[pos1+1 : pos2]
+	}
+
+	isLikelyTailerLine := lastLineKey != "" && unicode.IsUpper(rune(lastLineKey[0])) && strings.Contains(message, "-")
+	for i := 0; isLikelyTailerLine && i < len(lastLineKey); i++ {
+		r := rune(lastLineKey[i])
+		isLikelyTailerLine = unicode.IsLetter(r) || unicode.IsDigit(r) || r == '-'
+	}
+	if !strings.HasSuffix(message, "\n\n") && !isLikelyTailerLine {
+		message += "\n"
+	}
+	return message + tailerLine
+}
+
 // ErrInvalidMergeStyle represents an error if merging with disabled merge strategy
 type ErrInvalidMergeStyle struct {
 	ID    int64