Auto merge of #4944 - mathstuf:better-gitignore-template, r=alexcrichton

cargo_new: drop the trailing slash for target exclusion in Git

When `target` is a symlink (e.g., to keep build outputs on a separate
partition), Git will not match the `/target/` ignore to the symlink
since it is not a directory. Drop the trailing slash to support ignoring
`target` as a symlink.