- An identical expectation is included via the `skipped candidate` shared example in `spec/shared/method_matcher_behavior.rb`.

…iffs

* Avoids parsing source files that live outside the specified diff.
* Massively speeds up boot time on larger codebases and should be noticeable on all sizes.
  On `mutant` itself, when making a relatively targeted change (such as this one),
  boot time is reduced by about 0.7 seconds (~20% faster).
  On a larger project I work on, the speed up was 24 seconds or 2.5x faster overall!
  I left the original, full, subject filtering in place to simplify this change.
  This just short-circuits when the files we are interested in are not modified
  and `--since` is specified.
  * `mutant` run before on this change:
    ```
    Benchmark 1: bundle exec mutant run --zombie --since HEAD~1 --profile
    Time (mean ± σ):      4.192 s ±  0.350 s    [User: 8.567 s, System: 6.081 s]
    Range (min … max):    3.850 s …  4.960 s    10 runs
    ```
  * `mutant` run after on this change:
    ```
    Benchmark 1: bundle exec mutant run --zombie --since HEAD~1 --profile
    Time (mean ± σ):      3.535 s ±  0.213 s    [User: 6.890 s, System: 5.784 s]
    Range (min … max):    3.188 s …  3.784 s    10 runs
    ```
* Also reduces memory consumption significantly since the ASTs for all source files
  are never constructed.
  In my larger project on very small single subject change it reduced memory
  allocations by 69MB (80% reduction) and reduced "retained" memory by 25MB
  (99.4% reduction).