Fix overzealous file existance caching.

File existence check caching had a flaw where, if a file didn't exist,
we'd cache that it didn't exist, but if it was later pushed and then did
exist, we'd still think it didn't.

We now only cache when it exists. If it doesn't, we don't cache, and
will check again the next time get_file_exists is called.

Unit tests pass.