Use the text of pull-request as the squash commit's message (#13071)

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 #12365

Co-authored-by: Mura Li <typeless@users.noreply.github.com>

routers/repo/pull.go

@@ -440,7 +440,7 @@ 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())