specify dummy template dir when cloning

The solution I came up with for #5867 is to simply tell `git clone` to not use the global templates.  In fact, use *no* templates.  We create an empty `_templates` dir within the cache that we can tell `git clone` to use via the `--template` option.

Please let me know if, for some reason, we need global Git templates for anything, or there's a better directory/name to use.