Generated .gitignore should only ignore top-level target directory

[wesleywiser]

Fixes #1415

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

This is currently intentional actually due to the ability to have nested packages in one repository (each will have its own Cargo.lock and target directory). It's pretty easy to edit a .gitignore though, so this seems like a nice balance between defaults while still allowing to opt out of the default behavior.

Thanks for the PR though!

[wesleywiser]

No problem, thanks for taking a look at it!