Maybe copy should not ignore gitignore patterns by default

[sourcefrog]

people might be using the "ignored but added" pattern even inside their source tree, and so using it while copying would make an unusable copy. Maybe using gitignores isn't a good default.

Originally posted by @sourcefrog in https://github.com/sourcefrog/cargo-mutants/issues/450#issuecomment-2494042651

There is already an option for this, but it defaults to on.

https://github.com/sourcefrog/cargo-mutants/blob/main/src/main.rs#L189-L192

Probably, the main thing we care about is to not copy /target by default, because that might be large and often the contents can't be reused. However, we have a separate option for this: currently deprecated, but it could be un-deprecated:

https://github.com/sourcefrog/cargo-mutants/blob/c8d09ab4831742bdefcff29501ccaab0b563d220/src/main.rs#L259-L262

#494 suggests we could also have an option to copy /target.