Ignore only the target-dir or target-dirs that are in use

[passcod]

The default is currently just **/target/ (*/target/** before 9.0), which ignores all folders named target. This can be undesirable, so we could figure out where the target-dir is, and only ignore that. In the case of super workspaces (#205), there could be multiple target-dirs in play. Also, the Cargo config may affect their location.