Mention --with-cargo in tutorial or make it a default

[petrochenkov]

Without correct version of cargo any version of rustc that is older than few weeks will produce random build errors which will be counted as "regression found" by default.

It is pretty confusing when bisect-rustc tells that revision xyz produces an error and silently stops bisecting correctly, while cargo +xyz build doesn't produce any error when run explicitly.

[chrissimpkins]

As discussed in https://rust-lang.zulipchat.com/#narrow/stream/217417-t-compiler.2Fcargo-bisect-rustc/topic/unrecognized.20option.20json, we will transition to the --with-cargo behavior as default.

For my reference:

configuration source is here:

cargo-bisect-rustc/src/main.rs

Lines 96 to 100 in 82e234f

	#[structopt(
	long = "with-cargo",
	help = "Download cargo [default: installed cargo]"
	)]
	with_cargo: bool,

cargo-bisect-rustc/src/toolchains.rs

Lines 430 to 440 in 6baf139

	pub(crate) fn from_cfg_with_url_prefix(cfg: &Config, url_prefix: String) -> Self {
	DownloadParams {
	url_prefix,
	tmp_dir: cfg.rustup_tmp_path.clone(),
	install_dir: cfg.toolchains_path.clone(),
	install_cargo: cfg.args.with_cargo,
	install_src: cfg.args.with_src,
	force_install: cfg.args.force_install,
	}
	}
	}

install source is here:

cargo-bisect-rustc/src/toolchains.rs

Lines 234 to 244 in 6baf139

	if dl_params.install_cargo {
	let filename = format!("cargo-nightly-{}", self.host);
	download_tarball(
	&client,
	&format!("cargo for {}", self.host),
	&format!("{}/{}/{}.tar", dl_params.url_prefix, location, filename,),
	Some(&PathBuf::from(&filename).join("cargo")),
	tmpdir.path(),
	)
	.map_err(InstallError::Download)?;
	}

[chrissimpkins]

#98