Ignore only root target directory

Ignore only root target directory

We used to ignore all target directories, because it was common to
have multiple packages with different target directories in a single
repository. Now, when workspaces are here, such setups usually have a
single target, and we can .gitignore only it. It's useful because
sometimes you want to have a module named `target` in Rust.

If you use non-workspaced multi-package setup, you can create a
.gitignore with `/target/` for each package.