$ git clone http://repo.or.cz/w/i18n.git error: Can't lock ref rm: cannot remove directory `i18n/.git/clone-tmp': Directory not emp ty
At last I found the URL was a gitweb URL not for mirroring.
It's better git provides clear information like "This is maybe a gitweb URL, view it in web browser for more information", or gitweb can send a redirection response to git client. Even more, git-daemon also can send redirection response to tell git client that the service is offline temporarily and it can use an alternative