Implement better force update behavior that ignores no-ops

[mitsuhiko]

Today force update will typically report lots of changed snapshots even if they did not actually change. Additionally it generates pending snapshot files for inline snapshots which result in absolutely no changes. This changes the behavior to compare the serialized contents to the current ones and only write a change if there is one.

Fixes #333

[max-sixty]

Thanks!

I almost wonder whether force update should be default True now, given that they're not that volatile (no line numbers now). Or even whether a False option is that helpful.

We'll try making it default True in PRQL at the next insta release and see how that goes.