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 #450 (comment)

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:

cargo-mutants/src/main.rs

Lines 259 to 262 in c8d09ab

	/// Don't copy the /target directory, and don't build the source tree first.
	#[arg(long, help_heading = "Copying", group = "copy_opts")]
	no_copy_target: bool,