Before this patch, hg diff --stat will give an empty output. It will not show
the stat information. I debugged and found that the underlying code does not
return the diff header and due to that, other code paths fails to parse that as
I looked into why we don't return diff headers in quiet mode and found the
behavior is from 8f8bb77d560e70bcc95577e4dfa877df18d876ab which does not have
any mention about why it is done. We also show the diff headers in git, so I
think it's fine showing diff header in normal diff in quiet mode.