D6007: diff: make sure we output stat info even when --git is not passed (issue4037)

[Yuya Nishihara]

>   I looked into why we don't return diff headers in quiet mode and found the
>   behavior is from https://phab.mercurial-scm.org/rHG8f8bb77d560e70bcc95577e4dfa877df18d876ab 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.

Agreed. Flagged as BC and queued, thanks.