Always honor user's choice for "delete branch after merge" (#36281)

Fix #36280

Author: wxiaoguang

routers/web/repo/pull.go

@@ -32,6 +32,7 @@ import (
 	"code.gitea.io/gitea/modules/graceful"
 	issue_template "code.gitea.io/gitea/modules/issue/template"
 	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/optional"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/util"
@@ -1136,11 +1137,9 @@ func MergePullRequest(ctx *context.Context) {
 		message += "\n\n" + form.MergeMessageField
 	}
 
-	deleteBranchAfterMerge, err := pull_service.ShouldDeleteBranchAfterMerge(ctx, form.DeleteBranchAfterMerge, ctx.Repo.Repository, pr)
-	if err != nil {
-		ctx.ServerError("ShouldDeleteBranchAfterMerge", err)
-		return
-	}
+	// There is always a checkbox on the UI (the DeleteBranchAfterMerge is nil if the checkbox is not checked),
+	// just use the user's choice, don't use pull_service.ShouldDeleteBranchAfterMerge to decide
+	deleteBranchAfterMerge := optional.FromPtr(form.DeleteBranchAfterMerge).Value()
 
 	if form.MergeWhenChecksSucceed {
 		// delete all scheduled auto merges