Meta/CheckPush: fix the "does not match next" error case

When an early part of 'jch' does not match 'next', we showed the
whole diff between next and jch, which was not very useful.

CheckPush

@@ -72,10 +72,7 @@ then
 	else
 		echo "master..$next..next..$jch..jch..$pu..pu"
 	fi
-else
-	echo "git diff --stat next jch"
-	git diff --stat next jch
+elif match_next=$(git rev-parse 'jch^{/^### match next$}' 2>/dev/null)
+then
+	git diff --stat "$match_next" next
 fi
-
-match_next=$(git rev-parse 'jch^{/^### match next}') || exit
-git diff --stat "$match_next" next