Limit the output of cr-report --show-diff to surviving mutants (Feature Request)

[WillGibson]

It would be useful to be able to limit the output of cr-report --show-diff to surviving mutants.

E.g. cr-report --show-diff --surviving-only.

I would be interested in submitting a pull request for this if the Cosmic Ray maintainers want to agree on the acceptance criteria first.

[abingham]

That sounds good to me. If you can put together a PR I'll be happy to work with you to get it merged in.

[WillGibson]

We are also trying mutmut3, so it will like depend on available time factor based on which one we select.

Currently, mutmut is winning on speed and reporting, Cosmic Ray is winning on being supper easy to get working.

I may submit a pull request for fun anyway if I can find time.