Improve caching of individual requests

[armenzg]

When we fetch for a raw diff (or a changeset's metadata) and it is not on the local cache we don't get the ability to store the new data into the local cache. See https://github.com/mozilla/firefox-code-coverage-frontend/blob/master/src/utils/hg.js#L25

Also, if there's no local cache available then the individual fetch becomes the whole data stored in the cache.

This will not always be noticeable to the user, however, it will be on some edge cases.

In any case, I believe the code could handle individual fetches in a more beautiful way.