don't get confused by cache dirs we failed to create
don't get confused by cache dirs we failed to create

We were assuming that an existing cache dir meant the cache was already
created. But the problem with that is, we might succeed in creating the
dir but then fail to `git init` (in particular, if git isn't installed).
Instead, explicitly check for the files that git creates.

https://github.com/buildinspace/peru/issues/147

buildinspace/peru,oconnor663/peru,buildinspace/peru,oconnor663/peru