Print git info on test failure, for a particular test

For one CPAN Tester[0], the 'fork-point and merge-base are different'
test fails with unexpected upstreams, and it's difficult to investigate
further without getting more information about the git version and its
config.

[0]: http://www.cpantesters.org/cpan/report/3c155e5c-5392-11ee-8fb6-04536e8775ea

t/implicit_upstream.t

@@ -138,6 +138,13 @@ EOF
             $ok &&= Util::repo_state_ok($repo, $topic, $wants);
         }
 
+        # Help investigate an upstream mismatch that only happens for one CPAN
+        # Tester env.
+        if (!$ok) {
+            system('git version >&2');
+            system('PAGER= git config -l >&2');
+        }
+
         ok($ok, $name);
     };
     if ($@) {