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(())