Auto merge of #10810 - Diomendius:bugfix-zsh-completions, r=weihanglo
Auto merge of #10810 - Diomendius:bugfix-zsh-completions, r=weihanglo

Fix zsh completions for add and locate-project

Currently, trying to tab-complete either the `cargo add` or `cargo locate-project` subcommands results in output such as this:
```
cargo add _cargo:67: command not found: --default-features[enable the default features]
_cargo:68: command not found: --no-default-features[don't enable the default features]
_cargo:69: command not found: --optional[mark the dependency as optional]
```

This is because some line continuations are missing from `src/etc/_cargo`. This PR adds these line continuations.

This file gets packaged for Rust releases by https://github.com/rust-lang/rust/blob/acdcdfb61b7b472bfacbb8bb889bdf3204827f2e/src/bootstrap/dist.rs#L956 and Rustup ultimately places it in `~/.rustup/toolchains/*/share/zsh/site-functions/_cargo`. `rustup completions zsh cargo` outputs a script which sources this filepath.

The easier approach to testing this is probably to manually copy `_cargo` to its proper location under `~/.rustup`, assuming Rust is already installed via Rustup and Zsh completions for Cargo are already installed, but however you choose to install this, testing is as simple as observing that Zsh can correctly tab complete `cargo add` and `cargo locate-project`.

rustyhorde/cargo,rustyhorde/cargo,rustyhorde/cargo,rust-lang/cargo,rustyhorde/cargo,eminence/cargo,rust-lang/cargo,eminence/cargo,nipunn1313/cargo,nipunn1313/cargo,nipunn1313/cargo,eminence/cargo,eminence/cargo,nipunn1313/cargo,rust-lang/cargo,rust-lang/cargo