The Debug output of Command is very useful for showing to the user the
command that was executed when something went wrong. This is done for
example by rustc when invoking an external tool like the linker fails.
It is also overly verbose, since everything is quoted, which makes it
harder to read. Instead, we now first check if we're reasonably sure
that an argument is simple enough that using it in the shell wouldn't
need quoting, and then output it without quotes if possible.
Before and example output could look like this:
    PATH="/a:/b" "cc" "foo.o" "-target" "arm64-apple-darwin11.0"
Now it looks like this:
    PATH=/a:/b cc foo.o -target arm64-apple-darwin11.0