Determine whether to return instacgit.script by calling is_cgi()

handler.translate gets called with paths that have the /cgit/ prefix
stripped from them, causing instacgit to not work but spew error
pages, as it would pass back '/' as scriptfile:

Error response

Error code 403.

Message: CGI script is not a plain file ('/').

Error code explanation: 403 = Request forbidden -- authorization will
not help.