Add default commit messages to PR for squash merge (#20618)

Keep the same behavior as 1.16

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

routers/web/repo/pull.go

@@ -510,6 +510,8 @@ func PrepareViewPullInfo(ctx *context.Context, issue *issues_model.Issue) *git.C
 			return nil
 		}
 		ctx.Data["GetCommitMessages"] = pull_service.GetSquashMergeCommitMessages(ctx, pull)
+	} else {
+		ctx.Data["GetCommitMessages"] = ""
 	}
 
 	sha, err := baseGitRepo.GetRefCommitID(pull.GetGitRefName())

templates/repo/issue/view_content/pull.tmpl

@@ -395,7 +395,7 @@
 										'allowed': {{$prUnit.PullRequestsConfig.AllowSquash}},
 										'textDoMerge': {{$.locale.Tr "repo.pulls.squash_merge_pull_request"}},
 										'mergeTitleFieldText': defaultSquashMergeTitle,
-										'mergeMessageFieldText': defaultMergeMessage,
+										'mergeMessageFieldText': {{.GetCommitMessages}} + defaultMergeMessage,
 										'hideAutoMerge': generalHideAutoMerge,
 									},
 									{