fix wiki bugs (#1294) (#1338)

models/wiki.go

@@ -84,7 +84,11 @@ func (repo *Repository) LocalWikiPath() string {
 func (repo *Repository) UpdateLocalWiki() error {
 	// Don't pass branch name here because it fails to clone and
 	// checkout to a specific branch when wiki is an empty repository.
-	return UpdateLocalCopyBranch(repo.WikiPath(), repo.LocalWikiPath(), "")
+	var branch = ""
+	if com.IsExist(repo.LocalWikiPath()) {
+		branch = "master"
+	}
+	return UpdateLocalCopyBranch(repo.WikiPath(), repo.LocalWikiPath(), branch)
 }
 
 func discardLocalWikiChanges(localPath string) error {