Use the text of pull-request as the squash commit's message

Originally, it was filled by the commit messages of the involved
commits. In this change, we use the headline comment of the pull
request as the commit message when it is a squash merge.

Thanks to @zeripath for suggesting the idea.

Fixes go-gitea#12365

Author: typeless

routers/repo/pull.go

@@ -426,7 +426,7 @@ func PrepareViewPullInfo(ctx *context.Context, issue *models.Issue) *git.Compare
 			ctx.ServerError("IsUserAllowedToUpdate", err)
 			return nil
 		}
-		ctx.Data["GetCommitMessages"] = pull_service.GetCommitMessages(pull)
+		ctx.Data["GetCommitMessages"] = pull_service.GetSquashMergeCommitMessages(pull)
 	}
 
 	sha, err := baseGitRepo.GetRefCommitID(pull.GetGitRefName())

services/pull/pull.go

@@ -501,8 +501,8 @@ func CloseRepoBranchesPulls(doer *models.User, repo *models.Repository) error {
 	return nil
 }
 
-// GetCommitMessages returns the commit messages between head and merge base (if there is one)
-func GetCommitMessages(pr *models.PullRequest) string {
+// GetSquashMergeCommitMessages returns the commit messages between head and merge base (if there is one)
+func GetSquashMergeCommitMessages(pr *models.PullRequest) string {
 	if err := pr.LoadIssue(); err != nil {
 		log.Error("Cannot load issue %d for PR id %d: Error: %v", pr.IssueID, pr.ID, err)
 		return ""
@@ -549,34 +549,22 @@ func GetCommitMessages(pr *models.PullRequest) string {
 		return ""
 	}
 
-	maxSize := setting.Repository.PullRequest.DefaultMergeMessageSize
-
 	posterSig := pr.Issue.Poster.NewGitSig().String()
 
 	authorsMap := map[string]bool{}
 	authors := make([]string, 0, list.Len())
 	stringBuilder := strings.Builder{}
+
+	stringBuilder.WriteString(pr.Issue.Content)
+	if stringBuilder.Len() > 0 {
+		stringBuilder.WriteRune('\n')
+		stringBuilder.WriteRune('\n')
+	}
+
 	// commits list is in reverse chronological order
 	element := list.Back()
 	for element != nil {
 		commit := element.Value.(*git.Commit)
-
-		if maxSize < 0 || stringBuilder.Len() < maxSize {
-			toWrite := []byte(commit.CommitMessage)
-			if len(toWrite) > maxSize-stringBuilder.Len() && maxSize > -1 {
-				toWrite = append(toWrite[:maxSize-stringBuilder.Len()], "..."...)
-			}
-			if _, err := stringBuilder.Write(toWrite); err != nil {
-				log.Error("Unable to write commit message Error: %v", err)
-				return ""
-			}
-
-			if _, err := stringBuilder.WriteRune('\n'); err != nil {
-				log.Error("Unable to write commit message Error: %v", err)
-				return ""
-			}
-		}
-
 		authorString := commit.Author.String()
 		if !authorsMap[authorString] && authorString != posterSig {
 			authors = append(authors, authorString)