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