tests: new test for mq qdiff command
`hg qdiff .' is supposed to work but it is currently broken.
This test should help us verify when it gets fixed.
adding a
diff -r 107ba6f817b5 -r 310ce7989cdc a
--- a/a Thu Jan 01 00:00:01 1970 +0000
+++ b/a Thu Jan 01 00:00:02 1970 +0000
@@ -1,2 +1,3 @@ confuse str.splitlines
confuse str.splitlines
embedded
newline
+clean diff