Show new mutants relative to some baseline

[sourcefrog]

For example, in a PR, it would be interesting to see new mutants that are missed relative to the baseline, or mutants that are newly caught.

This could be done by reading a file from mutants.out in the old tree and doing some kind of intelligent diff. The line numbers are likely to have changed but we could probably match on the function name and mutation.

[sourcefrog]

Pretty much done in 23.11 by https://mutants.rs/in-diff.html