Prevent child cargo process from messing with our stdin

By default, `spawn` inherits stderr/stdout/stderr of the parent
process, and so, if child, for example does fcntl(O_NONBLOCK), weird
stuff happens to us.

Closes https://github.com/rust-analyzer/lsp-server/pull/10

crates/ra_cargo_watch/src/lib.rs

@@ -343,6 +343,7 @@ impl WatchThread {
                 .args(&args)
                 .stdout(Stdio::piped())
                 .stderr(Stdio::null())
+                .stdin(Stdio::null())
                 .spawn()
                 .expect("couldn't launch cargo");