diff: make sure we output stat even when --git is not passed (issue4037) (BC) 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 a diff. 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. Differential Revision: https://phab.mercurial-scm.org/D6007

tests: add test to demonstrate issue4037 `hg diff --stat -q --config diff.git=0` does not output anything whereas it should print the stat. This is a quiet old bug dating to 2013 and looking at code I think it exists it since 2005 or when --stat was introduced. The next patch will fix the bug. Differential Revision: https://phab.mercurial-scm.org/D6006