enhance: fall back to DEFAULT_TEMPLATE.md when style-specific template is missing (#38803)

Closes #38801

Introduce `DEFAULT_TEMPLATE.md` as the default message for all merge
styles.

https://gitea.com/gitea/docs/pulls/491

By the way, fix incorrect os.Expand usage for merge message & repo
template

---------

Co-authored-by: waterWang <waterWang@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/repository/create.go

@@ -91,7 +91,7 @@ func prepareRepoCommit(ctx context.Context, repo *repo_model.Repository, tmpDir
 		"CloneURL.HTTPS": cloneLink.HTTPS,
 		"OwnerName":      repo.OwnerName,
 	}
-	res, err := vars.Expand(string(data), match)
+	res, err := vars.ExpandCurlyBrace(string(data), match)
 	if err != nil {
 		// here we could just log the error and continue the rendering
 		log.Error("unable to expand template vars for repo README: %s, err: %v", opts.Readme, err)

services/repository/generate.go

@@ -24,6 +24,7 @@ import (
 	"gitea.dev/modules/log"
 	repo_module "gitea.dev/modules/repository"
 	"gitea.dev/modules/setting"
+	"gitea.dev/modules/templates/vars"
 	"gitea.dev/modules/util"
 
 	"github.com/huandu/xstrings"
@@ -86,12 +87,7 @@ func generateExpansion(ctx context.Context, src string, templateRepo, generateRe
 		}
 	}
 
-	return os.Expand(src, func(key string) string {
-		if val, ok := expansionMap[key]; ok {
-			return val
-		}
-		return key
-	})
+	return vars.ExpandShellLike(src, expansionMap)
 }
 
 type giteaTemplateFileMatcher struct {