Fix up Command Debug output when arg0 is specified.

PR https://github.com/rust-lang/rust/pull/66512 added the ability to set argv[0] on
Command. As a side effect, it changed the Debug output to print both the program and
argv[0], which in practice results in stuttery output ("echo echo foo").

This PR reverts the behaviour to the the old one, so that the command is only printed
once - unless arg0 has been set. In that case it emits "[command] arg0 arg1 ...".

src/libstd/sys/unix/process/process_common.rs

@@ -375,8 +375,12 @@ impl ChildStdio {
 
 impl fmt::Debug for Command {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{:?}", self.program)?;
-        for arg in &self.args {
+        if self.program != self.args[0] {
+            write!(f, "[{:?}] ", self.program)?;
+        }
+        write!(f, "{:?}", self.args[0])?;
+
+        for arg in &self.args[1..] {
             write!(f, " {:?}", arg)?;
         }
         Ok(())

src/test/ui/command-argv0-debug.rs

@@ -0,0 +1,24 @@
+// run-pass
+
+// ignore-windows - this is a unix-specific test
+// ignore-cloudabi no processes
+// ignore-emscripten no processes
+// ignore-sgx no processes
+#![feature(process_set_argv0)]
+
+use std::os::unix::process::CommandExt;
+use std::process::Command;
+
+fn main() {
+    let mut command = Command::new("some-boring-name");
+
+    assert_eq!(format!("{:?}", command), r#""some-boring-name""#);
+
+    command.args(&["1", "2", "3"]);
+
+    assert_eq!(format!("{:?}", command), r#""some-boring-name" "1" "2" "3""#);
+
+    command.arg0("exciting-name");
+
+    assert_eq!(format!("{:?}", command), r#"["some-boring-name"] "exciting-name" "1" "2" "3""#);
+}