FIX: Prevent cache corruption by moving before tags write.

A side effect of `xolox#easytags#write_tagsfile()` is that the entries argument is joined together. This then causes problems in `s:cache_tagged_files_in()`, where the filename element is addressed via `entry[1]`. When the entries have been flattened, this accesses a single character, and then the cache is corrupted.

To fix this, move the cache update before the writing of the tags file. This avoids the need to make a copy of the entries before flattening them.