This function will check a cache, it will only write the file if:
- The modified timestamps changed since last time we wrote something.
  This is useful to know if something changed by another tool or
  manually without diffing the full file.
- The contents changed.

Turns out that reading files and comparing them is fairly fast and there
is no huge benefit over only using the Stats of the file and keeping
track of that information.

Thanks @kentcdodds!