Choose subjects based on an AST diff when using `--since`

[dgollahon]

The --since behavior is great but it has a few rough edges. One of those is that trivial formatting changes (a far-reaching rubocop style change) can trigger a massive amount of mutations. Additionally, there are some issues where changing adjacent lines triggers unchanged subjects.

[mbj]

Yes so far mutant does a concrete syntax diff, but it should do an AST diff.

[dgollahon]

@mbj bumping this issue since it has been a recent pain point for us.